In the most basic terms, colleges are where students live (mostly in their first year). But it's also more than just accommodation, colleges offer opportunities for students to socialise and grow personally. They have facilities like a bar, gym, library, and common socialising rooms. Some may have dance and yoga rooms and music facilities as well. Each college has its sports teams for different sports which are labelled from A to whatever. A teams consist of the best players, B slightly lower than that and so on. So even if you're a beginner, you have a chance fo being in a college sport team and play with other beginners. There's something called a Junior Common Room (JCR) that each college has. It's basically a community of undergraduates in that college (irrespective of what year they are in) and you have access to support reps like international officer, welfare officer, events officer etc who help in everything happening around the college and you can always approach them if you face any problems. Your college will also have other support like financial, mental health and will be your first point of contact for any issues.
The colleges, based on their location, are divided into hill and bailey colleges. Bailey colleges are the ones closer to town and located around the Cathedral
(that makes it easier to distinguish them on Google Maps)- this includes Castle, Hatfield, Chads, St Johns and Cuths. Hill colleges are the ones located on South Road
(near the science site)- these include Marys, Grey, Collingwood, Van Mildert, Trevs, South, John Snow, Josephine Butler, Aidans and Stephenson. Hild Bede is neither a bailey or hill college- it's more on the bank of River Wear.
A formal is basically a fancy dinner that occurs in the college dining rooms where the dress code is usually black tie and some colleges wear gowns (like Harry Potter). The frequency varies from college to college- weekly, fortnightly, monthly. You can find content on this on @thedurhamstudent Instagram, Tiktok and YouTube channels so do check that out