Visa Waiver Program (VWP) is available as long as you do not stay longer than 90 days, don't work or study or apply to change your immigration status there. I am pretty sure it allows you to do business e.g. meet clients etc as long as you are not actually setting up a business or taking up work. You must register on the ESTA website; they recommend doing this at least 3 days in adevance.
However if you have ever been denied a visa and/or entry to USA, stayed longer than 90 days on a previous VWP visit or outstayed your visa you will have to get a visa to enter in the future.
Also worth mentioning, if for any reason you didn't hand back your green piece of paper which is stapled to your passport when you enter under VWP, they have no record of you leaving the USA and this could cause problems in the future; normally the airlines remove this from your passport when boarding but it can easily be forgotten about if you cross the border by land into Canada or Mexico and fly on home without returning to the USA. If this did happen, you can register your departure at a later date with the American Embassy (even after the 90 days) by supplying evidence that you returned home e.g. boarding cards, payslips.