In the strict definition of the word, an agent is a person who is responsible for booking shows for bands. Agents approach promoters about shows, negotiate contracts for live performances and basically make sure the wheels of a concert tour, or even a single show, turn smoothly. An agent will work with the promoter to make sure that everything a band needs will be at the venue, that there is an appropriate soundcheck period set aside for the band, and of course, what the payment will be for the performance and if accommodation will be provided by the promoter. Want only red M&Ms backstage? Your agent is the one who will make sure the promoter knows.
An agent working with small indie bands may step outside the strict role of an agent to occasionally promote shows. For all of this work, you can expect an agent to take a percentage of your income from your shows. Expect to pay between 10% and 15%, though 20% is not entirely unheard of. Note that this percentage refers to your base pay from the gig only, and does not include money you earned at the show through CD and merchandise sales.