Thy holy boltgun, sacred power armor, and pure, powerful soul, body, and mind.

As expected of an elite army, space marines are very well-equipped. A lot of the time, people forget this, but it's true. Let's take a closer look.
Standard kit for all proper marines is as follows: bolter or combat implement, boltpistol, power armor, krak and frag grenades.

Virtually every infantryman you can include will have this, usually with some slight variations. For instance, masters of the forge wear artificier armor, rather than power armor, and must pay points if they want an implement to beat people with. The biggest offenders of this rule are terminators, but in general, this is the gear you'll have, and can count on.

What this all means for you, the player, is that your infantry is capable of doing more than what's normally expected of simple grunts.

An example; ork boys are fighty, but don't shoot very well, and are powerless against vehicles.
A tactical marine fights adequately, shoots accurately, and hurts vehicles just fine.

This is partially because of his gear. Frag grenades let you charge things in cover, without going last. You also have a pistol, so can take a shot before you charge in. At range, bolters hurt light infantry, and vehicles don't like krak grenades one bit.

The ork boy in the example is cheap, but against an enemy at range, he dies very quickly. Orky bitz only provide a 6+ armor save, and will thus rarly be of any use. You, on the other hand, happen to be wearing power armor. This grants you a 3+ armor save - very good protection, all things considered.

However, as said above, the ork boy is indeed cheaper than the tactical marine - which brings us to the next part.

The boy is cheap, yes, but if he wants to accomplish anything, he needs backup. How much backup? Try eleven to twenty-nine more boyz. Compare this to your tactical marine, who needs four to nine buddies to accomplish things. If the boyz are reduced below 10 (easy to do), they start to falter, and no longer pose an actual threat to your army. A handful of boyz tend to die from bolter buttstocks before they even get to swing.

Everything taken into account and being fairly even, thirty unsupported boyz rushing ten tactical marines will likely lead to the boys dying, or taking heavy casualties before they even get to swing a single time.
Since your aim and gear are both so much better than theirs, they can't outshoot you. In fact, thanks to a very powerful, heavily underrated ability called combat tactics, they can't shoot you at all if their goal is mêlée, as you'll just fall back further from them, giving you more time to kill the boyz with your guns.

Should they ever reach you, your superior statline guarantees that you'll be going first, and that their return blows won't be effective. Even if they manage to win the combat, you're likely to hold - or fall back and regroup successfully, ready for more shooting, or a charge of your own. If you wish, you can even split your ten man squad into two units of five marines each, meaning the boyz now have two targets to worry about, and need to charge at least twice to win.

Tactical marines are weaponskill, ballisticskill, strength, toughness, and initiative 4. That's huge, and means every single squad you field can kill weak stuff in combat, at range, and soak any survivors' return fire.
They're also leadership 8, which is equal to - and sometimes better than - many armies' elites and characters.
Marines can regroup no matter how few of them are left, and will normally do so automatically at the end of their fall-back move. If you elect to do so, marine infantry can automatically fail any and all morale checks they're required to take - meaning you can disengage from losing combats and stalling battles, or fall back before combat even begins, due to enemies being greedy enough to shoot you.
All this in one package.

Veterans - of which you can field entire squads - have significantly more potent statlines, bring more gear, and have additional abilities over the above listed ones.

And now for the bad stuff.

You have limitations. Most are due to your army being formed of generalists. This means that you're capable of doing everything - fighting, shooting, dealing with armor, dealing with infantry, high reliability and durability, etc - but you're not the master of any given task, and specialists from other books will kick your marines' asses if given the chance. Thus, while you can't beat real combat units in combat, you can beat them at range, and enemy ranged specialists can be crushed in assault.

Your army is also small, and individual units are expensive in points. Care must be taken so you get the most out of your filled slots, spent points, and the army itself.

Next up, weapons.

Ed: If you have questions, ask away. Comments are highly appreciated, because they let us know whether you approve or disapprove of what's posted.