Think of it this way. A sample is a recording of just about anything you want. You save it on your hard drive as either an MP3, .aiff or .wav file. Usually the sample will be of something fairly short like a snare hit, but you also get longer ones like cymbal crashes that last as long as 15 seconds. On my hard drive for instance I've got groups of samples for about 20 different drum 'kits'. Everything from a cheap sounding garage band sound, to a compressed studio kit, to a huge stadium sound. There's also about 10GB of other samples I've got that are anything from blib sounds, to shakers, tweaks, synth stabs, orchestra hits... anything you want basically.
When I program the drums, first I have to figure out what sounds I need. So I'll usually scan through several of the 'kit' presets to find a drum sound that fits the song I'm working on. Next on the list is to put those sounds into your 'virtual drum machine'. I used to use the Cubase drum machine called LM-4 but found it limiting in that I could only have the drum machine playing about 14 different sounds per kit. I've recently changed to a drum machine called 'Battery' which is laid out almost like a spreadsheet.
There's enough places to put about 64 sounds in one drum 'kit'. Basically, I just go onto my hard drive and either import a premade 'kit' by choosing one ie, jazz kit.
The empty 'spreadsheet' spaces on the drum machine that were previously empty now automatically fill up to put a different sound into each space. A-1 would be the snare. A-2 the kick.
A-3 Cymbal. all the way up to H-9 or whatever.
You can then go back to your hard drive and still pick out individual sounds that you can just drag and drop onto the drum machine. ie, you find a .wav file of a guy farting and want that in your song...just select the file and drag it over onto one of the empty spaces in the drum machine.
Now whenever you click that space ya hear a fart.

Next up is to program the drums. You can either do it from scratch yourself or use premade patterns that have already been done for you in MIDI format.
You assign each one of those spaces on the drum machine to a note on your keyboard (or guitar synth), set your sequencer running and basically play drums on your synth by triggering each one of the sounds. You basically keep working on it until you come up with a complete song.

Another way of doing drums is to use loops. Which is basically a real drummer in a studio who's recorded 10 seconds or whatever of his drum playing.... and when you put it in the right program, it just loops the same thing endlessly. You can usually find several loops in the same style so that you can add fills, extra cymbal crashes and beginnings and endings to your song.

There's drawbacks to both ways though. MIDI drums, even using samples, tend to sound boring and most people can tell they're fake. Samples...same kinda thing. It's the same all the way through the song and any half-assed musician can tell when they hear the same roll 3 times.
I've usually found that the only way to even get close to a 'real' drummer is to combine samples, MIDI drums and playing a real kit yourself, then combining all of them to make up one full pallette of drums. We're talking about 20 tracks just for the drums alone... then you have to mix them down to stereo etc...
That's why it takes a week or more sometimes.

Try this song for an example.

http://www.schmange.com/NeonLies.mp3

The beginning up to where the vocals starts is a drum loop I got off a royalty free CD of drum loops. I've also added in a few little rolls and hits of my own on a real kit.
Where the vocal starts, it changes over to a midi kit, mixed in again with some real drums. Then it changes back to a loop and so forth. All the time, you're trying to come up with something interesting so people don't get bored.
Like I said, we're talking 20 or more tracks and about 8 days work. Then several more weeks of tweaking to get rid of little things that bug you.

Of course, once you've done all that work and you think it sounds amazing on your own equipment...you end up bouncing everything down to stereo on a cruddy MP3 for other people to listen to and it sounds like crap...but that's another story.

wow.. long post.