An ESTA is a prerequisite to (most) travel to the US if you are a holder a passport from a visa exempt country. The ESTA:
- Is required in advance of travel (at least 72 hours)
- Requires a fee
- Is valid for 2 years
In practice then this just seems to be a multiple entry tourist visa, with the only difference being (according to some sites) that visas are approved electronically instead of by an actual person. Most other visa free schemes I know of literally just require you to show up with the correct type of passport.
Then why is this considered not to be a visa? It just seems to be a justification for the visa-waiver program to not really be visa-free.