These products combine a NAT based hardware router with a wireless Access Point and allow both Ethernet and wireless clients to share files and printers and also an Internet connection. This means that you can't use two APs to wirelessly connect two non-wireless LANs together. This chart from Compaq's WL series reviewer's guide gives you some idea of what to expect, but like all manufacturer specs, is optimistic in terms of the actual effective data rate you'll see. It's not uncommon to have an under 50 foot indoor range in a typically constructed residence, and to have speed begin to drop off with a few walls between stations. You can also set up Internet sharing using a wireless card as the second network adapter.
Some manufacturers have software APs available, too.
You can combine wireless, HomePNA, and Ethernet networks. Go to this page for more info and example setups. Check this page for wireless networking technical articles. Go to this page for wireless networking help. Corp. All Rights Reserved.