Type an extension, with or without the dot, to learn what kind of file it is and what opens it.
The extension after the final dot in a filename is a claim about the file's format, usually right, sometimes wrong (renaming a file changes the claim, not the content, which is why this site's File Signature Identifier checks the actual bytes). The directory below covers the extensions people actually meet as attachments, downloads and exports, with the category, a plain description, and the common programs that open each. It is a curated reference, not an exhaustive registry of the tens of thousands of extensions in existence: obscure application-specific formats from one vendor's software are out of scope.
Safety note attached to the most-asked entries: an extension alone never makes a file safe. Executable types (.exe, .msi, .bat, .ps1, .jar, .scr) run code when opened, and documents can carry macros, treat unexpected attachments of any type with suspicion regardless of extension.
The extension is not in the directory, now what? Use the File Signature Identifier on this site to read the file's true format from its header bytes, then search that format name. Vendor-specific formats identify themselves there more reliably than by extension.
Can I just rename .heic to .jpg? No, renaming changes only the label. Conversion rewrites the data; the HEIC to JPG converter on this site does the real thing.
Why does a file have two extensions, like .tar.gz? Chained formats: a .tar bundle that was then gzip-compressed. Programs peel the layers in order, outside in.