A few of the most popular online chat services are Skype, Discord and IRC. Each platform functions a bit differently from the next, and some apply to specific situations more than others.
Skype provides a wide array of features that customers look for in a chat service, allowing users to create persistent profiles and maintain lists of contacts to chat with at any time. Skype offers text, voice and video chats, and it allows all to be used in group settings. Skype is available on mobile, and a person can call out to standard phone numbers and receive calls as if it were a normal phone as well.
Discord strives to provide services similar to Teamspeak and Ventrilo. Discord focuses more on the persistent chat room aspect of chat services, but it also offers voice chat. It is unique in that users do not have to start calls or be forced into voice chat rooms for these services; they may join in whenever and wherever they please. The service's app is very user-friendly and is actively being updated and improved upon with new features as of 2015.
IRC is a relic of the past in terms of the Internet, but it is still used by the technically savvy. While not relatively user-friendly, certain communities like to use Internet Relay Chat for communication. The service is a text chat and uses a sometimes spotty connection protocol, but its versatility (if the users know what they're doing) for more than just chatting has kept this type of chat alive since 1988.