"Chat" in internet terms refers just to TEXT chat - typing back and forth. But VOICE chat is for programs or sites that allow users to TALK back and forth, as well as or instead of typing.
Sites listed here let all visitors to the site talk to each other live (in turn) and many also have text messaging built in. To talk on any site one needs at least I.E.4, or Netscape 4.08, or AOL 4, or better, a working microphone or headset, and a very small browser "plugin" which should load automatically once when you first access the page.
The different systems used appear to be: Yahoo Chat, Excite(using LipStream), LipStream alone, Vocaltec Surf'n'Call, TelcoPoint using HearMe, plain "old" HearMe VoiceCreator, and "new" blue HearMe. (Some of these have since ceased - refer FAQ).