A Computer is a material used in numerous block types, and is usually found in any kind of mechanical or technological block.
Something unique about computers is that (in Survival Mode) they define who owns the block. When a block is being welded, whoever welds it past the computer layer is made the owner of the newly created block. This is indicated on the block's health bar with a blue line. This mechanic is also how you can take control of blocks not owned by you, or "hack" them. For more information on this, see here: Hacking.
Ship and Station part Recipes
These are ship and station part recipes that use Computers: