Mastodon has emerged as one of the fastest-growing and highest-profile options for people looking for an alternative to Twitter in the wake of Elon Musk's erratic changes.

The big picture: Mastodon says it had more than 1.8 million users active over the past month — more than triple the tally number it had on the day Musk took over Twitter.

• But Mastodon isn't simply a Twitter clone — and isn't trying to be. Here's what you need to know to get started.

Mastodon is not a company

It's a social media app that, like Twitter, allows users to share brief messages in real time — but, unlike Twitter, there isn't one giant system run by a for-profit company.

• Instead, Mastodon is a network of privately operated servers that run on shared open-source code.
• Mastodon has thousands of "instances" — individual servers — that users join as their home bases. These servers are "federated" into a network so that, once you've picked a server, you can readily follow posts from users based on other servers.

Mastodon isn't trying to clone Twitter

While much of Mastodon looks and feels a lot like Twitter, there are some key differences.

• There's no algorithm promoting posts that have drawn heavy engagement from other users. You see a chronological list of everything that users you follow have posted.
• There's no one list of trending topics across Mastodon, though individual servers can have their own lists. And while you can retweet ("boost") other users' messages, you can't "quote tweet."

Mastodon's content rules are decentralized

Each server's operator sets the terms for what kinds of posts are beyond the pale.

• Some welcome sexual content and some don't.
• Most have chosen to adopt fairly common prohibitions on hate speech, abusive behavior and misinformation.
• But it's entirely feasible for a server to, for example, welcome QAnon conspiracy theorists or white supremacists.

The intrigue: Mastodon allows other server operators to block servers whose practices and content they consider harmful.

• Users on a server that was widely blocked could communicate with each other, but not with most of the Mastodon world.

How to get started on Mastodon

Joinmastodon.org is a good starting point for getting the app and picking a server.

• As with Twitter, you can use Mastodon in any web browser, too.
• Tip: Set up your account in your browser first — it's easier and works better than doing so in the app.
• Don't sweat the choice of which server to join. You can move an account fairly easily from one to another.
• Tech journalist Clive Thompson has a good intro. Wired has a helpful guide for finding your Twitter follows on Mastodon, or use Fedifinder.

How Mastodon compares to Twitter

The service's lack of polish and additional layers of complexity have left many Twitter users feeling confused or lost when they hop over to Mastodon.

• Also, because Mastodon isn't a company, individual servers are often run by volunteers and there's little in the way of customer support beyond the help other users can provide.
• Some Twitter communities — among them, science Twitter and journalism Twitter — have shown heavy interest in Mastodon. Others, including Black Twitter and MAGA Twitter, appear to be holding off.

Be smart: Mastodon itself feels little of the pressure to scale up fast that drives most social media companies. It's not looking to sell eyeballs to advertisers or goose metrics for investors.

• This means that Mastodon has fostered a lot of micro-communities and developed a native nonconformist streak.
• Some Mastodon users are thrilled to see hordes from Twitter descending on the place. Others might be happier for their alternative dimension to grow a little more slowly.

Editor's note: This story has been corrected to note that Mastodon does offer trending topic lists, but they're separate for each server.