A music trade show (or music convention) is a networking and promotional opportunity for people in the music industry. They are attended by record labels that are looking for distribution and licensing deals or who have new music to promote, managers and agents who have artists to promote, distribution companies looking for new clients, and often unsigned musicians who are hoping to land a record deal (although this is often not a great idea: find out why). Music industry trade shows are also often attended by people who have some kind of new music business promote, like a new website or program geared towards the industry, a new music player and so on. Journalists also attend these events, both to cover the actual trade show and to check out new music.
Every music industry convention is different. Some are known for their networking opportunities, some are known for their panel discussions, some are known for their band showcase, some are known for being great for indies, some are know for being must-attends for the majors, some are genre specific - the list goes on and on. See below for descriptions of various music trade shows.
The set up of trade shows varies as well, but in general, the days are reserved for networking, meetings, and panel discussions. Often, there is an exhibition hall, where various music companies have display tables set up so they can promote their product and meet potential customers/partners. Music showcases usually happen in the evenings, at venues spread out across the host city, but sometimes showcases take place during the day as well.
Wondering if a music trade show is right for you, and if so, which one? These articles will help:

