If you have experienced IRC, it's basically that. You have smaller rooms (groups) and bigger ones. Every room has their active 1%, their own rules and customs and after a while, their own inside jokes etc.
Otherwise, it's a bit like reddit because you join groups that are usually about a certain topic / niche (general programming, JavaScript, Counter Strike, cooking, what have you) and it's also a bit like twitter because there's often a "hot topic" for a day or two with small things happening in between. Also because no one in their right mind treats those chats like actual group chats, as in, you don't read all the messages you missed. After a weekend i have at least 10k "missed messages".
This comparison is also cool because it reminds you to treat telegram just like reddit and twitter: it's public basically. Anyone can join groups you're in and log every message (there's plenty bots that do that) and they can see the contents of a public group even without joining. It's not encrypted by default and frankly, barely anyone uses secret chats, if only because ppl are on their desktop half the time anyway