IEEE 802.11a is an amendment to the 802.11 standard for wireless LANs. It is of of the specifications that is more commonly known as Wi-Fi.
802.11a uses radio frequencies in the 5 GHz band and supports theoretical throughput of up to 54 Mbps. The standard uses the same base protocol as the original 802.11 standard, but uses orthogonal frequency-division multiplexing (OFDM).
The first IEEE standard for Wi-Fi was released in 1997 and is known as IEEE 802.11. It had major shortcomings in that the maximum throughput was 2 Mbps. By 1999, two amendments were made to the original standard. 802.11a operated in the 5 GHz band and used OFDM, while 802.11b was still in the 2.4 GHz band and used DSSS.
Despite being superior in many ways, 802.11a never achieved the level of commercial success as 802.11b due to price. 802.11.b was cheaper, and got adopted as the de facto standard. It's more common in recent days to see tri-mode wireless routers with 802.11n and 802.11b/g. 802.11a isn't obsolete per se, it's just another example of how the business side of technology trumps performance.