If the seed isn't a 12 digit number (so if you're putting text) the generator just generates a random map (I may be wrong here, using the same seed in any other format may just change your spawn point). Using a 12 digit number, however, will give you the exact same world in the exact same spawn point every time. You can test this yourself: make a world with a seed of 123456789123, and this should be the first thing you see:
If it's not, then make another world with the same seed. Your two worlds should be identical. I've tried this 3 times, and have recieved the exact same world and spawn every time. If this has been posted already, sorry!
Disregard the above. It seems that as long as two seeds are the same, they will give you the same world. Whoopsie
That's odd; I restarted minecraft, and now it's giving me a different spawn every time I type the same seed in. I think that it's generating the same world each time, but it has a few preset spawn points, because I managed to get the spawn in the first picture once. I guess I was just insanely lucky to get the same spawn 3 times in a row. So we're getting the same world, but different spawns. That kind of throws my 12 digit idea out the window. Anyone want to see if you get the same world with, say, a 9 digit seed?