Choosing what type of service to use in Azure
I am planning to create a service that checks something on a website with an interval. Compares the results with the DB and sends a email out if changes has been made. No user interface is needed.
What type of service should I use for this type of application?
There's no single correct answer to your question. If you want to keep things really lightweight, you could use an Azure Function App with a timer trigger.
If the Function App model isn't right for you, you can deploy a scheduled Web Job to an App Service deployment.
For some scenarios, the best alternative is a virtual machine with cron / Windows Task Scheduler. For others, it's to write an app that comes with its own scheduler.
You can try to look at Azure Functions. That is probably the simplest thing you can do and it can run on timer. That seems quite relevant to your use case.
Azure Functions supports an event based on a timer using Cron job syntax. For example, execute code that runs every 15 minutes and clean up a database table based on customised business logic.