SVOD = Subscription VOD
You pay a subscription fee (usually by a monthly rate) for watching with no limits.
The prime example is Netflix.
TVOD = Transactional VOD
You pay each time for the content you choose to watch. Usually there are two options - rent and buy.
The prime example is iTunes.
AVOD = Advertising VOD
You are allowed to watch content for free, however you must watch advertisements.
The prime example is Hulu.
Many streaming services have a combination of two or even all three of SVOD, TVOD and AVOD models.