[Timber-WoIf]

(assuming here you want to know how they actually play...)
guard can bring a crippling amount of weapons to bear, but have close combat ability worse than most other races. That means, you generally do allot of standing and shooting. game turns will take longer because of the number of units you will have on the field. Also, you will have to be extremely careful deploying your troops, as it can be tricky overlapping your guns effectively, once again because of the number of units you have to deploy. Your reliance on heavy weapons as well as cover will make moving some of your units risky and costly. If playing guard taught me anything, its how to set up my guns to get optimum fields of fire.

of course, i play a stand back and shoot guard army. but i haven't been able to figure out any other way to play them, save an Armored company.

space marines are definitely very versatile. You can field just about any style of army you want with them. The only drawback is their popularity. it seems most everyone plays space marines.