This command enables spell checking:

:setlocal spell spelllang=en_us

This enables the spell option and configures it to check for US English.

Unrecognized words are highlighted with one of the following:

	SpellBad	word not recognized			|hl-SpellBad|
	SpellCap	word not capitalised			|hl-SpellCap|
	SpellRare	rare word				|hl-SpellRare|
	SpellLocal	wrong spelling for selected region	|hl-SpellLocal|

Vim only checks for spelling; it does not perform grammar checks.