Which sentence is more formal?
The principal said that a lot of students graduating from Oakville High School got into the colleges they wanted to go to. The principal announced that half of Oakville High School's graduating class had been accepted to their top-choice colleges.
Formal writing is used for essays, business letters, and reports. The following types of informal language should be avoided in formal writing:
Type | Examples
slang | cool, awesome
idioms | knock your socks off
conversational language | gonna, kinda, yeah
abbreviated language | ASAP, FYI
overly simple or imprecise language | he got some stuff at the store
contractions | can't, won't
Contractions are not as informal as the other types, but they should be used sparingly in formal writing.
Compare the following sentences. The first is informal. The second is formal.
Informal: Yeah, ostriches can't fly, but they're awesome runners.
Formal: Though ostriches are flightless, they are remarkably adept runners.

The second sentence is less formal. You can tell because it uses overly simple or imprecise language (said, a lot of, got into).
The first sentence uses more precise language, so it is more formal overall.