Approval letter based Visa on arrival is that which is applied after arrival at another country's airport. Dedicated counters are located there where you have to be present with required documents and fees and apply for the visa on arrival. The term VOA means that you can arrive at the destination airport and be granted a visa right thereafter a short process(provided you satisfy the norms).
While eVisa is an electronic visa that is generated online at any country's evisa website.
You have to upload the required documents and pay the fees online. A receipt will be generated and after 2 or 3 days, evisa is sent to your mail if approved. You have to print it and carry it on the journey.
An eVisa is an alternative to a paper-based visa issued by the embassy. Instead of applying for visa at the embassy, the Immigration Department has now launched an e-VISA program, which enables you to apply for a visa online.
For example: If you get eVisa for EU country, then you can choose any destination in the EU for your first arrival. However, in ALBVoA, you have to be in that specific country where you get a visa after arrival. After that, you can travel inside the EU.