Everything in the 3D world is an entity. There are many different types of entities, with different properties. Press N in the editor to see the entity properties.
The worldspawn is a special entity which includes all of the walls/floors/etc which do not need to be seperate for any special reason. All the non moving walls/floors/etc are world spawn. Doors however are not part of the worldspawn.
There can only be 1 worldspawn. Every map must have a worldspawn.
Brushes are those solid blocks in radient. They are the basis of every level. Brushes can be part of the world spawn or part of a seperate entity, such as a door or a func_group.
Patches are part of the worldspawn like brushes, but they're a mesh already, not seperate blocks. They use a different texture editor which you can access through shift + s.
Patches can be a variety of shapes, most comonly: a plane, cylinder, cone, or end caps (ie, the ends of the cylinder).
The game wont read the .map file that is produced by the editor. You need to compile it into a different language known by the game's engine. This is done by the compiler.
There are 3 phases of the compile: