Cories, aka corydoras catfish, are small catfish, you can read more from their profile on this site. They are very interesting to watch, especially if you have sand in your tank; they love to play in it. There are many varieties; Bronze, albine, Panda and peppered(what I have) are the most common. All are basically the same, coloring is the only differnce.
Otos, AKA Otocinclus catfish, are also small catfish, but instead of whiskers they have a small sucker mouth. They stay small, are extremely peaceful, and do an excellent job with algae. They can be a bit tricky at first; many people find they're hit-or-miss - they often die soon after being introduced to the tank. However, my experience hasn't been this way; having purched three in the past, all of which lived for some time. Additionally, people say that even if some do die when new, the ones who survive are usually pretty hardy.