They often require ATP to actively transport materials across the membrane against a concentration gradient.
They are usually not particular about what types of chemicals they will allow to cross the membrane.
They can act as a channel, allowing the transport of ions across the membrane.
They may be receptor proteins that bind specific molecules from the surrounding solution, which triggers endocytosis (i.e., receptor-mediated endocytosis).
They often facilitate diffusion of molecules across the membrane that otherwise would diffuse too slowly to be of use to the cell.