If you’re on Mastodon, you can follow other Mastodon accounts of course, but you can also follow accounts from many other types of Fediverse server such as PixelFed, PeerTube, Friendica, BookWyrm, OwnCast and many others.
Also, the following process is so seamless that you probably won’t even notice you are following accounts outside Mastodon! They will look just like any other account on your timeline, and you will appear the same way to them. Fediverse servers always display content in their own style, regardless of the style used on the server the content came from.
If you’re using Masto through the website, you can see what an account really looks like by going to the account’s profile page, clicking ︙ and then “Show original page”. This will open the account’s public profile page on its home server.
Following a non-Mastodon Fediverse account is easy, it’s exactly the same process as following a Mastodon account: if you can already see their profile just click Follow, or if you have their account address paste it into the search box and it will display their profile.