PuTTY is a free open source client for telnet emulator supporting several network protocols including standard telnet, SSH, SCP, SFTP, rlogin, and a couple others. It is developed by Simon Tatham and first published on 1999-01-08 for Windows, but has since been ported to Linux and Macintosh. Officially, the name "PuTTY" doesn't stand for anything, although "TTY" has long been an abbreviation for "teletype," which is partially what the program emulates.
Because it is both free and open source, the program sees a lot of use. For what it does, it is quite good, however the interface is pretty awful. Part of this is from supporting so many protocols and options, but part of it also is trying to compact the UI into such a small area. It's definitely not designed for amateurs, but, most of what you would do with it is not the sort of thing an amateur would do.
I started using PuTTY in the mid-2000s at my work as a way to make changes to our Web servers. After becoming mildly familiar with it, I began using it to communicate with my own personal Web servers as well. Although I still find the program intimidating, it's the best free one I've found.