On Windows systems, all files usually have three or four-letter extensions after the file names. For example: .jpg, .txt, .docx, etc. Executable files, in particular, have an .exe extension. In Linux, files do not need an extension, even executable files, although typically documents and images still have them.
Text files in particular do not need the .txt extension in Linux, and executable files can have extensions but do not usually. This is partially due to the differences in file system structure. Nearly all executable files are stored in /bin, /usr/bin, /sbin, or something similar, rather than in various directories scattered through the drive.
On a server, many of the files you work with might not have extensions, but those files that are accessed from a website will (i.e. .html, .php, .pl).