Which sentence is more formal?
Museum members get ten percent off the things they buy in the museum gift shop. Museum members receive a ten-percent discount on all items purchased at the museum gift shop.
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 (get, things).
The first sentence uses more precise language, so it is more formal overall.