Informal and Formal Writing
Look at Abe Lincoln from the waist up. He looks formal—dressed like a president. Now look at him from the waist down. He looks pretty informal, dressed like a pioneer. He was both, of course. He wouldn't wear his stovepipe hat to split logs, and he wouldn't go barefoot to Congress!
Writing can be informal or formal as well.
Informal writing: We gonna eat soon?
Formal writing: When is dinnertime?