Survival Mode is one of the two modes that Minecraft offers. In this mode you have to survive with the resources you have gathered and collected. Once the day is over night comes and mods will start to appear shortly after night has fallen. (Click the "Mods" link on the left hand side of the page for more information about mods.)
Survival Mode Menu
This is the menu where you can choose which mode you want to play in. In the picture above it shows that survival mode has been selected. For survival, and creative, you can name your world anything you would like. Once your done click the "create new world" to start playing in Survival Mode.
When the new world starts this is your menu. You have your inventory and 9 boxes so you can have 9 items in it whenever you need it, you don't have to go to your inventory to retrieve it.
The above picture shows your inventory when you open it. (Unless you have already gathered resources.)
In order to survive you must be prepared. Create weapons, armor, and the most important thing, a crafting table. You must find resources and go mining in order to survive the Minecraft world. Survival mode is all about testing your survival skills with mods, biomes, and many more features.
The screen you recieve when you have died in the game and have no hearts remanining (try to avoid this!)