Telegram is a messaging app, if you install it you’ll be presented with a list of people from your contact list that have it installed as well. You can message these people through the app, and that’s about it.

Telegram also supports group chats, and several apps/etc use these group chats for support and discussion. There are also some group chats used for illegal activity, but you can’t discover these through the app itself. You would have to get in touch with these groups on another website, and be given an invite link to their telegram chat. The only thing that makes telegram more popular for illegal activity (afaik) is that you can have private group chats without government/telegram oversight.