Yes, Tasker is an automation app developed by a third party. It’s a very popular app for building automations on Android - lots of neat things like being able to react to various features and events on your phone and automate various tasks on your phone.
The SharpTools Tasker Plugins are basically a bridge between Tasker and SmartThings. Enabling you to control your SmartThings devices from Tasker or read data from your SmartThings devices into Tasker.
As Jake mentioned, the SharpTools Tasker Plugin license is tied to your Google Account via Google Play.
The SharpTools.io platform on the other hand is a single license that works across all of your devices and enables Dashboards and the Rule Engine. The dashboards are web based which means they run in any modern web browser regardless of the device type. The Rule Engine is also accessed and configured from your browser and is a complete automation platform (eg. you can build your triggers, conditions, actions all within the Rule Engine).
Edit: I also just wanted to clarify that the Tasker Plugins are specifically designed for interacting with SmartThings from Tasker. If you’re looking to automate things in SmartThings or Hubitat, I would recommend checking out the SharpTools Rule Engine. The Tasker Plugins are really neat if you have a specific need for integrating with Tasker and events on your phone, otherwise most people find the SharpTools Rule Engine better suited for their needs.