APK (Android application package) is the file format used to distribute and install application software for the Android operating system. It can only be used with the Android OS.
JAR (Java Archive) is a archive file format typically used to distribute a set of Java classes. These files are done using the ZIP file format and have the .jar file extension. They can be used on multiple devices including those that have the Android OS.
You can convert apk to jar, and vice versa.
APK files are saved in a compressed .ZIP format and can be opened by any Zip decompression tool. You can either rename the file extension to ".zip" and open the file
Jar file compresses and contains other files. Jar files are very easy to open. There are many programs that will open jar files. In fact, any program that can open zip files can open jar files. The jar format is identical to the zip file format. Java runtime environment is needed to open Jar files.
To open APK files on your computer, additional software is needed.