Both 802.11a and 802.11g offer data rates up to five times faster than 802.11b (54 Mbps versus 11 Mbps), and they also promise to transmit signals more cleanly and with fewer lost packets. This means an improvement in overall throughput. Throughput, compared to simple data rates, is the actual rate achieved after you factor in hardware limitations, network congestion, buffering, transmission errors, and so on.
This is possible thanks to improved modulation technologies in the guise of Orthogonal Frequency Division Multiplexing (OFDM). OFDM is a more efficient means of transmission than Direct Sequence Spread Spectrum (DSSS) transmission, which 802.11b uses.
802.11a and 802.11g also offer more throughput than 802.11b. Table A-1 compares throughput for the various network technologies.
Notice the throughput hit 802.11g access points (AP) take when 802.11b clients are associated with them. This is because 802.11g was designed for backward compatibility with 802.11b, which nicely leverages an existing investment. When an 802.11b client associates, the slower traffic takes up more air time at a slower speed to communicate with that client. Because of this, the 802.11g clients have to wait longer.