ATAs are usually the size of a small network hub/switch. The basic ones come with an ethernet port and a phone jack. The idea is simple. You plug your regular phone into the phone jack instead of plugging it into the wall, and you plug the ethernet port into your switch. The device itself registers on the IdeaSIP network. When calls come in, your phone rings and you can answer it. When you want to make a call, you pick up your regular phone and dial. ATAs are a convenient way of getting a SIP-capable phone, and they work with any traditional touch-tone phone, from the multi-handset cordless models to the simple, cheap, corded ones.
ATAs come in many shapes and sizes. Some have features like PSTN passthrough, which allows you to plug your ATA into the phone jack in the wall, and will let normal calls come into your phone as well as SIP calls. Some have built-in network switches. Some are tiny and portable enough to take on a trip. We've tested two main brands here, but there are many others.