FOSS gamedev and creative worlds

What is the difference between register_node and register_entity? also are there any rgister other than the ones above and tool and craft?

Register_node is registering a regular node into the game
Register_entity registers an entity, for example a mod or something that moves
Yes there are many more register_ in the game. Read the lua_api.txt in the doc folder

This page should help explain the registry functions, and where the registered data can be accessed:

http://api.minetest.net/page/view/view/registry