African American Vernacular English/Grammar
Generally, AAVE is similar to Mainstream American English (MAE), but some grammatical features are unique when compared to the standard. For instance, a double negative (It ain't worth nothing, Now I'm not no professor but I know what you're talking about) does not make a positive, but rather signifies emphasis. Certain prepositions/verbs, e.g. "be", "gonna"(going to) can signify aspect, leading to phrases such as "He be working" that would normally be considered off.
Is AAVE "Bad Grammar"? edit
No. Though its rules differ from those of Standard English, they are internally consistent and one can tell if a grammatical concept is being misused. Though historically its teaching has been met with much controversy, the grammatical features are agreed not to be flaws in the speech of individual speakers.