Seeds are referred to the "Seeds" option when you are generating a new world. Implemented in Minecraft Beta 1.3, seeds make it so you can choose what type of world you spawn on. Minecraft players have the option at the beginning of the game to name their world, and add a "seed". This can be a series of numbers, letters, or symbols. A Seed works as a base number for a random generation of a map, in older versions when maps were randomly generated the seed was picked randomly. Now in Beta 1.3 Players are able to chose the seed and create the same map result.
Some notable seeds include...
1 - A series of small islands, some which include small pools of water, and large amounts of sand/clay.
2 - Lots of stone mountains/canyons visible.
3 - Flat islands of sand.
4 - You are spawned in the middle of a desert.
5 - Flying stone cliff above water.
100 - Desert.
666 - Big desert.
1234567890 - Large flat grass areas.
Mayor - Floating land structure
It has been confirmed that the same seed will result in the same map, the spawn location however is still random.
Some of these seeds were found because of this thread! Thank them :) - http://www.minecraftforum.net/viewtopic.php?f=3&t=189706