Using Mastodon externally
You can browse and interact on Mastodon from external apps or websites.
Remote interactions on another Mastodon site
When you are browsing a remote site powered by Mastodon, clicking on any of the interaction buttons will load a dialog that will redirect you to your local site.
Signing into a client app
You can use your Mastodon account to sign into any app that implements the Mastodon API. A list of such apps can be found at https://joinmastodon.org/apps.