Sometimes called a sex pilus (plural: sex pili). Up to ten of these structures can exist on the bacteria. Some bacterial viruses or bacteriphages attach to receptors on sex pili at the start of their reproductive cycle.
Despite the name "sex pilus", this has nothing to do with sexual reproduction or mating, nor is it the bacterial equivalent of a penis; such misnomers are used quite frequently in describing the process, and while may prove useful in understanding underlying concepts are misleading nonetheless.
A pilus is typically 9 to 10 nm in diameter. The sex pilus allows for the transfer of bacterial DNA from the bacteria with the pilus (donor) to the recipient bacteria. Through this mechanism of genetic transformation, advantageous genetic traits can be disseminated amongst a population of bacteria. Not all bacteria have the ability to create sex pili, however sex pili can form between bacteria of different species.
It is an extension of the cytoplasm and used for attachment to surfaces (and is then called a fimbrium) and conjugation with another cell of the same species.
Basically... a bacteria's dick.
Your dick is the size of a pili.