Years ago it used to be 'How do I open a PDF file?' and the online population banded together to put those little Adobe Reader icons everywhere -- honestly you still see them around and they could use an extreme makeover -- I mention PDF only because that's a question you hardly hear anymore, I think everyone and their 90-year old grandma now know how to open a PDF file so perhaps those funky 80's styling icons do work? (If you are one of the 4 people on the planet who doesn't know how to open PDF please download Adobe Reader)
I'm getting off track -- the question you mainly hear today (apart from which video player do I use to open this crazy commercial my buddy sent me) is how to a open 7z files (and also how do I open RAR files but most people are getting that now).
Well to answer the first question a 7z is a type of compressed file created by 7-Zip - these files with the .7z file extension are compressed files and in concept just like ZIP & just like RAR -- except someone has used 7-ZIP because they are perhaps a little more cutting-edge (or at least want to appear to be ;) so instead of using ZIP & RAR, which are pretty well industry standards they've used something a little less common. Why? You may ask. 7Zip, albeit less common is a fine file compressor, in tests it compresses files better than most other compressed file formats (i.e. it makes them smaller and therefore quick to email/share) and it often uses less resources to do it. It's also open source which is nice as people can #1 implement support for 7-ZIP in other applications and #2 you also know it's going to be free forever in some shape or form.
Now to answer the original question, if someone sends you a file with .7z at the end just download and install 7-zip and then reply to the sender asking why do they have to be a rule-breaker...?
(If someone has been a bit more civilized and sent you a ZIP or a RAR then it's usually best to open ZIP with WinZip, and if you are wanting to open RAR best to download WinRAR.)