Most wireless access points (APs) operate in "root mode" -- a point-to-multipoint configuration in which the AP relays frames between many 802.11 stations and an adjacent Ethernet LAN.
Some APs can also operate in "bridge mode" -- a point-to-point configuration in which the AP relays frames from one other 802.11 bridge onto an adjacent Ethernet LAN.
Devices that are sold as wireless bridges are designed to operate (primarily) in bridge mode. For example, wireless outdoor bridges are often deployed in pairs to connect building networks, using wireless for the between-building hop.
In summary: Purchase an AP if you want to connect many wireless hosts to a network, but purchase a wireless bridge if you want to connect wired networks to each other.