Anime types represent the medium for which an anime was first released.
Anime that has no type set yet. Generally for anime that hasn't been released yet.
Anime that is first released as a series on TV.
Original Video Animations (OVA) are animated films and series made specially for release in home-video formats (e.g. VHS, DVD, BluRay).
Original Animation Discs (OAD) are animated films and series made specially for release as an item bundled with something else, such as a manga, light novel, visual novel, etc.
Anime that is first released as a movie in theatres.
Anime that doesn't fall into any other category.
Anime that is first released as streaming media.
Original Net Animations (ONA) fall into the Web category, rather than the OVA category as the name may otherwise suggest.
Anime that is first released as a one-off, (generally) non-recurring episode/movie on TV. A TV special can have multiple episodes.
A notable differentiation from a normal TV series is that TV specials generally are not serialized. Further, if Japan/China/Korea specifies that it is a TV special, it is probably a TV special.
Anime that is a music video.