Real-time voice communications over the Internet. It's like IRC; but it's not typing and reading; it's talking and listening. It's like a telephone; but software, a sound card, a microphone, and speakers are used instead.
Communication using a microphone. See video chat
Audio message exchange between on-line users. It has many applications. All you need is a computer connected to the Internet and a program for this purpose which can be found on-line and free of charge.
Instead of using an Instant Messaging service to type your messages back and forth to a friend, some services like QQ offer a voice-chat option where, if you are both online and both have QQ, speakers and a microphone (or a headset), you can actually physically talk to the other person as if you were on the phone but using the Internet instead of a phone line. The bonus of voice chat is that other than your standard bandwidth-usage or Internet dial-up costs, the call is free – perfect for international conversations.