APK stands for Android package kit. This is a file format that is used by the operating system in Android for the installation and the distribution of middleware and mobile apps. The files are actually analogous to the other packages like APPX for Microsoft Windows or the Debn kind of packages within debian based kind of OS such as Ubuntu.
So as to come up with the APK file, one needs an Android program for Android to be compiled and then all parts packaged into a single file that contains the code of the program. This can be files like .dex files. The APK file also has to have the manifest, certificates, assets and resources files. Just like any kinds of file formats that you may know of, the APK files are allowed to have any kind of name that is needed. The only thing that has to remain constant is the end which should be .apk.
They are a kind of archive file and found in the zip format packages that are based on JAR format .apk is the filename extension.
One can install the files in the devices that are Android powered just the way we install software on the PC. When one downloads and then installs an application on Android from official sites or other unofficial sites, they are actually installing APK ion the device.
It is possible for a person to install APK files directly in devices. This can be done without necessarily downloading them from any network. You can use a desktop computer and use different communication programs.
You can also use a process that is referred to as side loading. Usually, this ability to install from sites that aren't official or from the file manager or direct from a desktop comes disabled because of different security reasons in many of the Android devices that we have. You can enable this by simply changing the unknown sources settings within the menu.
Why install an APK file
Usually, APK files get leaked before time and this allows you to have an easy access to many new features that would not have been availed to you. There are also some apps that certain regions can actually restrict. Having the APK files allows one to be able to bypass the various carriers so as to have the very latest updates on Google. This can take a lot of time to arrive on the network. In the Android device you are using does not have access to the play site, then installing the apps may require having the APK files and it may be the only option that you have.
When using this method, you need to be careful not to end up with apps that aren't legally acquired. There are services that may allow the download of apps that are pirated. Do your research so as to avoid breaking laws. Find a downloader that is well known for its efficiency.
Always ensure that you use a site that you can really trust since there are APK files that can have some malicious software.