Someone new to the study of UFOs and alien activity, might be confused by the terminology used with nonchalance by those of us who have been dedicating ourselves to the subject for years. As a way to remedy to this difficulty, I have compiled a small glossary of the most important terms and ideas. Keep in mind that this glossary is not extensive, and as a matter of fact, it is an important skill for any ufologist to check various sources to any claim. Anything that is lacking from this glossary, you will find elsewhere, and this is just one of the many sources that will be helpful for you to consult.
UFO: Unidentified Flying Object
UAP: Unidentified Aerial Phenomenon
Grey/Gray Alien: Species of alien of which there have been sightings of various levels of credibility. The vast majority of extraterrestrial sightings in the USA is of Greys, however this is not true in other countries, and it is perhaps true that the usage of Greys in science-fiction has inspired some fake sightings as well. Grey aliens have a humanoid shape, but with significantly smaller stature than the average human adult, and a slender figure. They are hairless and have grey-coloured skin. They have large heads compared to the rest of their bodies, and big, dark eyes. They have no noticeable ears, noses and reproductive organs, but they do have nostrils and mouths.
Nordic Alien: Species of alien of which there have been sightings of various levels of credibility. They are also referred to as Pleiadeans and Venusians. Sightings of Nordic Aliens were especially common up until the mid 20th Century, but have since diminished. Nordic Aliens are said to have appearances similar to that of humans. They tend to be taller than humans, with many of them reported to be over 200cm in height, and are frequently described as blond and blue-eyed. Their physical characteristics are said to be similar to those of Northern European descent, which inspires their name.
Reptilian Humanoid Alien: Species of alien of which there have only been sightings of low levels of credibility. Reptilians are said to be anthropomorphic reptiles, frequently lizards, who are sometimes said to have shapeshifting abilities. Theories that claim that prominent human figures are reptilians in disguise are unfounded and often seeded in antisemitism. However, there have been sightings and descriptions of similar aliens outside of conspiracy theories, but none particularly credible.
Flying Saucer/Disc: A term used to refer to a common description of an extraterrestrial aircraft. The term is mostly out of use nowadays, but can still be heard. A flying saucer is a type of aircraft that is round and disc-like in shape, and often described as moving in a motion similar to that of a saucer across water.
Close Encounter: A term made famous by the Steven Spielberg movie "Close Encounters of the Third Kind", but originally coined by UFO researched Allen Hyneck. A close encounter describes an event in which a human is a direct witness to a UFO. A close encounter is then classified differently depending on the involvement that the person has with the UFO and, if present, the beings inside of it. A close encounter of the first kind happens when the witness sees a UFO from close enough to be able to see details of it, but only sees it and doesn't interact with it at all. A close encounter of the second kind happens when in addition to witnessing a UFO, the person notices some sort of physical effect as a direct consequence of what they witnessed. This can include effects on the environment or on the person themselves. A close encounter of the third kind happens when an animated entity connected to the UFO is also present; this is generally assumed to be an extraterrestrial being. Finally, a close encounter of the fourth kind happens when the human witness is abducted by the beings in the UFO.
MIB: An acronym for 'men in black', used to refer to government agents tasked with keeping news of extraterrestrial activity on Earth secret. While they are sometimes claimed to have futuristic technology they can use to stop witnesses from talking or even remembering their close encounters, the most reputable accounts of people who interacted with the MIB involve witnesses being offered payement in exchange for silence, being harassed, being threatened or sometimes assassinated to keep quiet.
Ancient Astronauts: A term used in connection to a theory that claims that in ancient times, the Earth was visited by intelligent extraterrestrial life which shaped early human history. Often, these "ancient astronauts" are credited with the building of such things as the Pyramids which are thought to be too complex to be built only based on the at the time presumably limited human knowledge.
While even scientists of such credit as Carl Sagan have flirted with the idea, no real credible proof exists that aliens significantly shaped early human life. Ancient engineering knowledge has been proven to be more than enough to construct megalithic buildings. The rest of the evidence provided by the theory is mainly based on findings in religious texts. While certainly true that interactions with beings more intelligent than us might have led humans to worship aliens and eventually, throughout their history, change the shape of the alien into that of gods, demi-gods or aliens, it is also purely speculative. Therefore, while an interesting and popular theory, we have to say that the "Ancient Astronauts" thesis isn't backed up by a lot of evidence at the moment.
Area 51: The name commonly given to the highly classified United States Airforce base officially named "Groom Lake". The US government has been extremely secretive about the nature of what is done in Area 51 for the entire history of the facility, and only acknowledged the existence of it as recently as 2013. A lot of theories have emerged about the nature of Area 51's work, with the most believable ones involving a relation to the remains of the Roswell aircraft crash, and possibly more evidence of extratterestrial life and its contact with Earth.