Also, can you explain ESTA to me?
ESTA is a an agreement with various countries for its citizens to enter the states for 3 months without a visa. Many people I know use ESTA to work in a grey-zone
Really. US immigration is more perverse and frustrating than you can imagine. You might be fine, but just because someone said 'I did this and it was OK' doesn't mean it will be. If you aren't sure, take legal advice.
Also, point of information, ESTA isn't a visa. They did it on the Visa Waiver Program (VWP). ESTA is part of the process of entering on the Visa Waiver Program
Moreover the visa waiver is used extensively by technology companies, investment banks and consultancies when having foreign employees work out of their US offices for short periods (upto several months).
Is it a legally airtight option ? - no. Is it in practice what everyone uses ? - yes.