Telnet: Dating back to the late 1960s, Telnet is a service used over an internet connection to allow a remote computer to log into another computer or device. Windows systems often allow for this kind of remote access (e.g. Remote Desktop) through a Telnet client.
Telnet is known as a client-server protocol, used to establish a connection to Transmission Control Protocol (TCP); this is sometimes also referred to as TCP/IP. Web browsers, for instance, use TCP to connect to servers on the internet and send email and files. Telnet uses this protocol in order to send information over the internet.
For the most part, Telnet and similar services are used to "remote" into a device and may be used to provide support for technical issues. However, these types of services can lead to major security issues, especially if used over an open internet, so today SSH (Secure Shell) is preferred.
The main issue with using Telnet for a remote log-in is that none of the data sent over the network is encrypted. This leaves any computer open to security vulnerabilities, especially because of password security. Telnet also does not require any authentication between two hosts, making it easy for information to be intercepted by a third party. There are also several vulnerabilities with Telnet daemons, or programs that run in the background.
Using a Secure Shell, which was first released in 1995, allows for greater encryption of passwords and public key authentication to ensure that the remote computer is not a malicious third party.
However, there are still ways to download Telnet clients, which today are typically combined with Secure Shells. Microsoft offers its own Telnet client; PuTTY is a free Telnet/SSH client.