Aliens fall into 3 main categories, extraterrestrial, sub terrestrial and ultra terrestrial.
Extraterrestrial come from outer space, sub terrestrial come from underground and ultra terrestrial come from other dimensions.
The most reported type of alien are the Greys. they are small about 3' to 4' in height, they have large heads with large eyes and small mouths, they communicate telepathically. they come from the star system Zeta Reticulum. They are responsible for most of the abductions.
Reptilian's are larger then Greys and come from Alpha Draconia. They are considered to be hostile and use the Greys as slaves to do there work experimenting on humans. They come in various forms from green ones without wings to white ones with wings (Draco). The Draco are in charge and are rarely seen they are the only ones to have horns. this type of reptile is the subject of the film "The Moth man Prophecies".
The other commonly reported alien is the Nordic Humanoid. these vary in size to normal size humans to giants known as ELS or the elder race. they are thought to be less hostile then the Greys and Reptiles.
Other types of alien include the Chupacabras and Men in Black although the exact nature of MIB is not known they might be government agents or aliens or robots or some inter dimension phenomenon. The Chupacabras are found in Brazil mainly in Puerto Rico. They are commonly known as Goat suckers because they drink the blood of farm animals mostly goats. they have wings and large eyes but unlike the Draco don't have horns on there heads but bony plates.
They may be alien or a hybrid from bats, monkeys and possibly aliens.