802.11b (WiFi)
The most established wireless LAN technology, it's also the most affordable. Allows wireless connections up to 300 feet from an access point, and can easily be added to existing wired networks. With speeds up to 11 Mbps, performance is comparable to a standard wired Ethernet network. Industry standard 802.11b products are easy to find and compatible with each other.

802.11a802.11a
provides a bigger pipe for data and supports more simultaneous users. Ideal for deployments where speed and bandwidth are important, 802.11a networks can run at up to 54Mbps and support more users per access point than a Wi-Fi solution.

Are they compatible?
802.11a devices do not talk to 802.11b devices, but there are certain access points (usually called dual access points) that enable the use of both types of networks simultaneously. The speed for each user is determined by what kind of adapter the user has in his computer or wireless device.