In many cases you may want to install apps that are unavailable to you directly from android play store. The unavailability may be due to geographical restrictions, phone incompatibility or some other reason or you may want to install an app on your Kindle Fire, but the app is not available on Amazon appstore. whatever is your reason, the workaround is to install using an apk file.
An apk(application package installer) file is an installable file for Android OS very much like what an msi file is for Windows.
Instagram is one such app that is not available to me since the play store says it is not compatible with my phone. I installed the instagram using an apk file and it works without any issues. In many cases, though the play store says the phone/app is incompatible, they work perfectly well after installation.
It is not advisable to install using an apk file unless you know the risks associated with it. Installing an incompatible app may result in erratic behaviour of the device, crashing etc and some times the device may even stop working. The best and safest way is to install apps that are available to you from your appstore/playstore. If you still want to go ahead and install using an apk file, here are several ways you can obtain an apk file for an application you want.
1. Downloading from the app developers website. This is probably the safest(after play store) as you are directly downloading from the application developers website. Check if the app you want is available for download from the developers website. One such app is Viber messenger, this can be downloaded from Vibers website.
2. Evozi's apk download tool. Evozi has a free tool using which you can download the apk of the file you want. Evozi's tool can be accessed at http://apps.evozi.com/apk-downloader/ . As per the site, they are able to generate 400 apk files a day. This is a easy tool which worked everytime I wanted to download an apk file.
Go to the above link and in the text box fill the "Package name or Google Play URL" and click on "Generate Download Link" button. The tool will then extract the apk and it will display the link to the apk on screen. You can download the apk using the link and install it.
Google Play URL is the url that is shown in your browsers address bar when you visit the particular application page on Google Play Store. For eg. https://play.google.com/store/apps/details?id=com.instagram.android is the URL for instagram. Package name is the part after "=" in the url. In the above example, 'com.instagram.android' is the package name.
3. Using an apk extractor. For this, you need to have an Android device which already has your required app installed, say your friends phone. You can then extract the apk from this phone, transfer it to your device and install. There are many apk extractor tools/applications available on play store(search for apk extractor). Choose and install an apk extractor app on the phone which has the required app. Open the extractor and choose the application for which you want apk extracted. The tool then extracts the apk file from the installation(this does not extract user data, passwords etc). Transfer the extracted apk file to the device on which you want to install the app and install.
4. Search google for the apk file and download it, this will be my last option. Many dedicated android websites host apk files for most of the apps. You can download an available apk from one of these sites and install. Download from a website that is popular and trust worthy.
Once again, resort to apk files only if it is unavoidable. It is best to install only those available on the play store.
To install downloaded apk file, check this link http://www.faqtory.in/v1/60/how-to-install-an-android-app-using-apk-file