An ESTA covers short visits to the United States for a limited set of purposes. Knowing where the limits are matters, because using an ESTA for the wrong kind of trip is refused at the border, not at the application stage.
What you can do with an ESTA
Tourism and holidays
Visiting family or friends
Business travel: meetings, conferences, negotiations, training
Transit through the United States on the way to another country
Short medical treatment
What an ESTA does not allow
Working for a US employer or being paid by a US source
Studying on a course that leads to a qualification
Journalism or reporting work
Staying longer than 90 days on a single entry
Living in the United States, or entering repeatedly to stay in practice
If your trip falls into any of those, you need a visa. An ESTA cannot be extended into one, and being refused entry once makes every later application harder.
An approved ESTA is not a guaranteed entry
This surprises people, so it is worth saying plainly. An approved ESTA means you are authorized to travel to the United States and to present yourself at the border. It does not mean you will be admitted.
The final decision belongs to the US officer who meets you on arrival. They can ask about the purpose of your trip, how long you intend to stay and how you will support yourself, and they can refuse entry even with a valid ESTA in the system.
In practice this is rare for travellers whose trip matches what they declared. It becomes a risk when the answers given at the border do not match the application.
Being honest on the application matters
The application asks direct questions about your history and your intentions. Answer them truthfully, even when an answer feels awkward. A truthful answer that leads to extra checks is far better than a false one, which is treated as misrepresentation and follows you for years.
If an honest answer worries you, tell us before we submit. We can explain what happens next, and in some cases the right advice is to apply for a visa instead.