The Basic Rate set is the rates that all clients that want to associate with a given access point must support. For backward compatibility with 802.11b clients, the Basic Rate set is generally 1, 2, 5.5 and 11Mbps. This information is transmitted by an access point as mandatory rates in the Supported Rates element of various management frames.
Some very old 802.11b clients may only be able to associate with APs advertising a 1, 2 Mbps basic rate set. This is why some APs allow you to change the Basic Rate setting.
The Transmit Rate setting is used to set the fastest rate that an AP or wireless router will send data. It can be used to force a lower rate in order to trade off speed for more reliable connection in WLANs where many clients are operating at low signal levels.
Standard 802.11g supports rates of 6, 9, 12, 18, 24, 36, 48, 54 in addition to the 802.11b rates of 1, 2, 5.5 and 11Mbps.