The most common relative pronouns in written English are that, where, which, who/whom/whose, and why. Correct use of these words helps a text to feel more formal.
The bird that is sitting in the tree is black and white.
This formulation is inefficient for the user, who is forced to specify a large number of variables.
People whose car does not meet the standard could decide to use other means of transportation.
[Note that the correct use of relative pronouns in formal academic writing can differ from spoken or informal English, in which the relative pronouns are often dropped. The following example is not grammatically incorrect, but it is less formal.
The bird sitting in the tree is black and white.]