Most tickets, as referred to in legal terms, serve as bearer coupons. Meaning: the bearer (or one in possession) of the coupon will gain admission to the specific event stated on it. A registered ticket, on the other hand, becomes registered to a specific owner and only that owner can redeem it.
A ticket agent serves as the sole agent between the venue and promoter to provide ticketing services and limited advertising for an event (i.e. Ticketmaster, etc.). The venue will sell tickets as well, through its box office; however, the ticket agent will handle the majority of all sales through its various outlets and call centers.
A ticket broker, as the name implies, serves as a broker of tickets. Just as a stock broker buys and sells stocks, a ticket broker buys and sells tickets. More importantly, ticket brokers specialize in the secondary market of premium tickets. A reputable firm will carry a wide range of tickets to many entertainment events offering anywhere from good to excellent seats; however, in the event of a sellout most brokers will expand their seating range to include more variety. This provides the customer with many choices so that anyone may sit in a preferred seating location at a reasonable market price.
In addition to the usual fare of buying and selling tickets, most brokers will provide advance information about upcoming events (something ticket agents many times cannot or will not do). They also will provide venue seating charts. And will even provide for travel and accommodation needs, but usually at an extra cost. For their long-distance customers an established broker will generally provide a toll-free number, who in turn can order with a major credit card, and then have their order shipped to them via a major overnight service (i.e. FedEx, UPS, USPS, etc.). For their customers without a credit card, most brokers will offer the option to have tickets delivered by C.O.D. (cash on delivery).