The IEEE 802.11 standard uses these terms, without precisely defining them, as follows.
Bandwidth is the difference between the lowest and highest RF frequencies used, typically stated as a number of millions of cycles per second (MHz). Non-overlapping adjacent WLAN channels are typically centered every twenty MHz. Although the frequencies in use don't cleanly end at the edges of the channels, it is convenient sometimes to speak of the channels being twenty MHz wide.
Data rate is the number of bits per second the physical layer carries during a single frame transmission, including all protocol bits at all layers, typically stated as a number of millions of bits per second (Mbps). The HR/DSSS PHY has familiar data rates of 1, 2, 5.5, and 11 Mbps.
Throughput is less precise and its meaning inferred from the context. Throughput is a measure of bits per second actually carried by a service, typically stated as a number of Mbps. Considering the physical layer, throughput typically includes MAC protocol bits and MAC payload. When considering the data link MAC sublayer, throughput typically includes LLC protocol bits and LLC payload.
Sometimes throughput is measured by the number of bits in a large file divided by the number of seconds it takes to move it across an otherwise unused WLAN between a wireless client station and an Ethernet station. The difference between this measure and the nominal data rate for the medium is accounted for by inter frame spaces, physical layer headers, data link layer MAC sublayer headers, data link layer LLC sublayer headers, IP network layer headers, TCP/UDP transport layer headers, MAC contention including deferrals to other transmissions and interference, and MAC retransmissions. Under good conditions these typically absorb nearly half the data rate.
I hope this helps. Thanks. /criss