The demo file is playable by anyone who has tf2 installed. When you record it, it goes into your tf2 directory, and can be played from the console by typing "playdemo
demoname"
If you upload it somewhere we can download it, we'll be able to see exactly what you record. We've caught many people this way, thanks to our awesome community.
I have the following in my steam profile to try to help others, it seems relevant here:
If someone is breaking a rule in the server and it is something that occurs over a period of time (mic spam, griefing, friendlying, etc.) it is a good habit to record a demo
FIRST before making a report or using the in-game /report command. Information on recording a demo can be found here:
http://wiki.teamfortress.com/wiki/Help:Recording_demos and here:
https://www.skial.com/threads/collecting-evidence-in-tf2.47782/
Once you have a demo, upload it somewhere an admin can easily download it; there are plenty of file sharing sites around. Just make sure you don't need an account to download, we aren't going to make a million accounts for all the file sharing sites people might decide to use. Now you have evidence to put in the report thread! If you made an in-game report, you'll have to come to the forums and comment on the thread started by our reporter bot.
If someone is breaking a rule and it's something that is immediate and does not require a demo (illegal spray, racism in text chat, spamming, etc.) a screenshot will suffice. Upload it somewhere we can easily see it, again there are a million photo sharing sites out there, and same as with the demo make sure we don't need an account to see the picture. Same as above, come to the forums and put it in the report thread (preferably inline).
The upload of choice for demos seems to be mega.nz or dropbox.
I hope we can help you in the future and hope to see you in a server soon!