FTPS is FTP using the SSL protocol for encryption. This is different from the SCP/SFTP family of protocols which use SSH as their transport tunnel. You will usually use the same client programs for scp and sftp (WinSCP for instance; SFTP is an upgraded version of SCP), whereas you usually use a web browser or web Download manager (like filezilla) for FTPS. FTPS is web-based, whereas SFTP is based on secure shell protocols pioneered on *NIX systems.
This article describes the differences nicely: http://www.codeguru.com/csharp/.net/net_general/internet/article.php/c14329/FTPS-vs-SFTP-What-to-Choose.htm
Edit: To expound as requested:
FTP is a core Internet protocol that goes back to 1971 (with a number of updates over the years). FTP is primarily focused on content distribution and the SSL just provides security from eavesdropping and illicit modification. Most browsers do require an external plugin, for a few reasons but the most prevalent being that almost no one uses FTPS anymore, and last time it was popular, web browsers were almost exclusively HTTP only, and everyone used a separate FTP client. At the time FTP was most popular and FTPS made the most sense, HTTP was not capable of handling file downloads well; not until HTTP\1.1 was published in 1997 (and updated in 1999). FTPS use has been waning for a very long time now, and is usually used in niche circumstances these days.
The SSH family is a set of protocols focused on server administration and remote access to the servers processing capability, rather than simple content distribution. it would allow privledged users of a system to connect to a shell to perform work on the server itself, and many file management tasks related to that work involve transferring files between the localhost and the server, which is why SCP and eventually SFTP were developed. Imagine you need to install an update to your apache instance on a remote server. you ssh in, use scp\sftp to upload the update package, and then execute it to install. Quite a different work flow than you can achieve with FTPS.
I hope that helps clarify.