If you have a Windows machine, you may need a sound card, microphone, and speakers. If you have a Mac that doesn't have a jack, you will need a serial Microphone such as Sound Advantage or MacRecorder. If you use a modem, you will need a PPP or SLIP connection. A 14.4k connection will provide adequate sound but a 28.8k connection greatly improves quality. You may notice a delay using some applications however with good connections this delay will range from .01 to .05 seconds.
Some applications allow for full duplex conversation and others allow only for half duplex.
Full duplex conversation is similar to a typical telephone conversation. Both speakers can talk at the same time and each will be able to hear the other as they do. In half duplex conversation, only one person can speak at a time. If both people speak at the same time, they will be unable to hear each other. Most new Macintosh computers and many UNIX stations come equipped with full duplex sound cards, most PC's do not.