Pili are tiny, hollow projections. They are used to attach bacteria to surfaces and are not involved in movement.
Conjugation pili (SEX pili)
Conjugation pili found only in certain group of bacteria, attach two cells and may furnish a pathway for the transfer of the genetic material DNA. This transfer process is called conjugation.
Attachment pili or fimbriae
This structure help bacteria adhere to surfaces, such as cell surfaces and the interface of water and air.