Army selection is very important with Marines, because they have the ability to be very shooty, very flexible, or very Close Combat orientated, and so it important to decide what you want, rather than ending up with a force that is neither here nor there. For this part of the page I am just going to run through the capabilities of the various marine units.
Force Commander/Librarian/Chaplain: All are powerful in cc and have access to a wide range of wargear. For smaller battles the chaplain is probably the best option as they come with a 4+ inv and a Power Weapon and is the cheapest; his special abilities are also very handy. Equipped with a Bolt pistol, jump pack and terminator honours you get a very nasty little character indeed. Librarians are best suited to large battles as to give them the typical Librarian wargear costs a bunch. when choosing Psychic powers choose carefully. Something like Fear of Darkness may look brilliant, but not for a charcter who wants to assault, as you may use it and find that there is no longer anyone in assault range! Commanders now have one major advantage over other charcters. the whole army may use his leadership as long as he is alive. That is a very powerful special rule, but bear in mind it may just make your opponent more keen to kill him!
Veterans: Considered by many to be a waste of points, veterans have three important advantages over normal squads, they have a higher leadership, they can take ccws as standard, and they have the option to take terminators honours. This gives you three main options for Veterans. A support unit with higher leadership. All out assault with ccws and termie honours. Or an extremely flexible unit, with termie honours, bolters, ML and Plasma gun (A cc vet sarge will also enchance this unit). The fact that they have Veteran Skills makes them even more dangerous too. You can use them as an elite anti tank unit (Lascannon and plasma gun + Tank hunters) a shock assault unit which has the potential to do A LOT of damage (Special CCWs + Furious Charge), or an advanced fire base (infiltrate).
Terminators: Tough and packing real punch in shooting and cc, with the ability to move and fire all weapons, all traits which make termiators an endlessly useful unit. Against any enemy, terminators can be outfitted to deal with them. Facing heavily armoured troops? A largish squad with lightning claws and Furious charge will deal with them . Against nids the best possible combo is two heavy flamers and a bunch of powerfists to deal with the big gribblies. Against Tank Heavy armies, missile lanuchers/assault canons and chainfists, along with Tank hunters. Under the new rules, assault cannons make a truly terrifying anti tank weapon. with four shots at str 6 (7 with Tank hunters) and benefiting from the rending rule; put two of these babies in a squad and there is nothing they can't take down. Two such squads took down a Baneblade in three turns. You get the point. Add in the option to deepstrike in any mission or ride to war in the toughest tank around, and you have a nice unit.
Dreadnaughts: Good guns, good armour, good in cc. What more is there to say? Except that the most effective combination for flexibility is a Twin Lascannon and Heavy flamer, whilst Assault cannon and storm bolter is good against swarms and the twin las/ML combo for busting tanks. Making them Venerable can help keep them alive, but if you take a Veteran Skill as well they start to get really expensive.
Tactical Squads: The backbone of any Marine army. I find large squads work best as they take a lot of killing, but smaller squads do have their merits. For cc, give the squad a melta gun/flamer and a Vet sarge with either PW or PF, and mount them in a rhino. For shooty I would recommend ML and plasma gun, and then either ten men, or six men and a Razorback for added mobility and firepower. What ever happens, at least 20 Basic squaddies are a must IMO.
Scouts: Again large squads are the way to go, they simply won't last otherwise. Two possible roles for scouts: close combat, use them to pick on isolated units or in support of other units. They are surprisingly effective in cc though, lots of attacks and a decent statline, they just aren't as tough as normal marines. Or shooting, I'd recommend sniper rifles and a HB/ML, as your marines can provide the bolters; make the most of their abilities, and sniper rifles are really useful in large numbers.
Assault Marines: Endlessly useful, they are mobile and pack a lot of punch in cc, almost compulsory in a Marine army IMO. They can be used to spearhead an assault, support a large assault, counterattack, go tank hunting when equipped with melta bombs and so on. Two things to be careful of: Power armoured assault troops, make sure you get the charge and that support is on hand, chainswords just don't cut it against power armour. And secondly, make sure you take as many as you can in a squad, otherwise if they take any casulties prior to gettig into assault then their effectiveness is dramatically reduced. As Assault marines with Melta bombs are now cheaper than Assault marines without Melta bombs used to be, it seems churlish not to take Melta bombs, unless you are really stuck for points. Or playing Tyranids of course.
Bikes/Speeders: Fast and with good firepower, these are good for harrassing the enemy. Bikes are nice and tough so they are a good hold up unit. Charge a few bikes headlong into an enemy unit, with guns blazing and you should hold them up for a turn or two. Also their Turbo boost Special Rule makes them ideal for slipping around behind te enemy and attacking Support units. Speeders are just a real annoyance, make sure you keep moving them fast to keep them alive as long as possible, if left alone units of three with HBs can inflict absolute carnage against horde armies, while even a single LS with a Multi melta can make back its points several times over with just one shot. (Like when I took out a chaos Landraider on turn 1, see the Grand assault bat rep for details!). On the other hand your opponent can waste several turns trying to kill these cheap vehicles while the rest of your army does its stuff.
Predators: The predator tank is extremely flexible. I personally like my tanks to specialise, so i usually go with Autocannon and HBs, or three lascannons, but going for a more flexible option is good too, particularly when going to tournaments. A twin Lascannon with two HBs is the best flexible option IMO, as under the TVR it can move and fire all of its guns. A predator can make a huge difference but they are likely to attract a lot of anti tank fire so be careful how you deploy them.
Whirlwinds: Cheap, long range, no LOS required, and lethal against horde armies. Whirlwinds are the way to go. They can really disrupt an enemy plan by putting big holes in key units, and causing pinning. Whirlwinds also have a unique ability: they can lay minefields mid battle. Used in this way, a couple of whirlwinds can really disrupt an enemy advance, by constantly dropping minefields just in front of your opponents units, forcing them to walk through them and suffer the consequences or go around them, slowing them down.
Vindicator: Very useful for assaulting enemy fortifications. Can put big holes in even the most heavily armoured of units, good for clearing obstacles and for destroying bunkers too. A must have in cityfight battles, and a good choice in most just for sheer scare factor. A useful upgrade for this tank is the Power of the Machine spirit , this can allow it to fire regardless of whether its stunned or shaken, which will really annoy your opponent.
Devastators: These guys are lethal. Make sure you always give them extra marines to soak up casulties, and specialise. Don't go for the "one of each weapon" type squads, is just not worth the points. Nothing beats 4 Missile launchers for flexibility, four HBs for anti troop, four plasma cannons for anti Marine, or four lascannons for anti tank.
Landraiders: Heavily armed and armoured, and the ability to carry either normal Marines or Terminators. What else is there to say? (Damn costly though!)
This is just a brief overview of what each unit type can do. Knowing what a unit can do is vital when choosing an army. Simply think about what you are going to have to achieve in a mission against a particular opponent, and pick units that can do the job. Its no use taking a Whirlwind against a Marine army, or a Predator annihilator with three LCs against Orks. They simply won't achieve enough.
Now, onto deployment.