Evisas are issued by the foreign governmental agency and vary depending upon the country. Some visas (such as Australian ETA's) can take as little as 30 minutes. Others can take longer, more like 6-7 days to process. That being said, electronic evisas are much faster than paper visas due to the lack of need to send documents, stand in line, and go through paperwork. Its the most efficient way to obtain a visa for your trip.
Visas are issued based on who issued your passport. Most countries categorize your visa need into the following categories.
E-Visa is the easiest way to get permit of entry into and travel within the territory of chosen country for the purpose of short stay. All you need to do is to complete the online application form and pay with your credit/debit card instead of applying through Country Embassy. After receiving your Visa through email, print it out and bring it along when you travel to chosen country. Evisas are available for 20+ countries and growing.