The meaning of these abbreviations can be found on the Shareware List page. The links on this page mostly refer to the homepages of these concerning applications. From there on it should be easy to try to locate a download link. A filename extension is the mostly 3 letter word that you see after the period in a filename. Since Windows 95, extensions can be larger than 3 characters.
Extensions are usefull for determening the type of a file.
There must be thousands of extensions. Some are well-known and used very often, others are used just once by a single program. Programmers are free to make up their own extensions. On this site I try to make an inventory of extensions, sorted in alphabetical order. You might think this site is usefull if you encountered a file somewhere and you don't know what to do with it.