What does employment at will actually mean for an employee?

I've recently (5 months ago) signed and employment-at-will contract with a largish company. I start in 2 months. Given different scenarios, I want to figure out not only what this legally means, but what it entails as your possible options when looking for other jobs.

I haven't begun to work for them, but I'm getting interviews with other companies now (which I didn't have at the time) which look much more enticing. Keep in mind I haven't started at the first company yet, only signed a contract.

1) Am I obligated to tell the people interviewing me anything? What should I tell them? Why?

2) Let's say I get a better offer from another company -- what do I tell the one I signed for?

It means that both you and your employer have no legal obligation when it comes to terminating employment. You're free to leave whenever you wish, with or without notice. Although professionalism and respect usually lead employees to give two weeks' notice and assist in the transfer of duties. Your employer may also choose to terminate your employment at any time and without warning.

You have no obligation to tell other companies you're interviewing with. In fact, I wouldn't. It's an odd situation and won't reflect well upon you if you've taken a job - not yet started - and are still looking. A prospective employer might be concerned that you'd do the same to their company if you were extended and accepted an offer from them.

If you do get a better offer and choose to take it, tell them just that. Be honest. What you absolutely should not do is play counter offers against either company. I've been there before and will never do it again. It isn't good for anyone and can create a very hostile working environment.