Android is a mobile operating system developed by Google and licensed to phone manufacturers at no cost. Android manages the devices hardware and software resources to provide a platform for apps to use.
There are currently 9 versions of Android, each named after sweets. The newer versions offer more features and improved security. Currently, the most common versions, newest to oldest, are Oreo (v8), Nougat (v7), Marshmallow (v6), Lollipop (v5) and KitKat (v4.4).
Each version of the operating system provides a set of features to apps, this is known as the API level (Application Programming Interface). For example, Android 7.1 uses API level 25 (i.e. on this device an app can’t use features from API level 26 – apps are limited to the older set of features).
Google provide updates on a regular basis, mainly for security vulnerabilities. The level of update installed is known as the Security patch level and is a date. It is important to ensure you have fairly recent security patch updates.
Security patch updates are typically delivered to phones through OTA (Over The Air) updates from the phone manufacturer or your carrier. Typically you will be prompted by your phone when an update is available for download and installation.
Similarly, OTA is typically used when the next major version of Android is available for your phone and you will be prompted by your phone if you want to download and install the update. For example, upgrading from Nougat (v7) to Oreo (v8). The device manufacturer will determine if they will support the latest version of Android for your phone. You can check with the phone manufacturer if a major Android update is available.