chmod

chmod 400 file - Read by owner
chmod 040 file - Read by group
chmod 004 file - Read by world 

chmod 200 file - Write by owner
chmod 020 file - Write by group
chmod 002 file - Write by world

chmod 100 file - execute by owner
chmod 010 file - execute by group
chmod 001 file - execute by world

To combine these, just add the numbers together:
chmod 444 file - Allow read permission to owner and group and world
chmod 777 file - Allow everyone to read, write, and execute file