No warning is required. In practice it depends mostly on the size of the company and the size of the layoffs... I think most companies would like to layoff without warning, but that's not always practical. It also depends on if they are hoping to reduce headcount in general, in which case they may make a general offer of some money if employees leave... this isn't generally a viable strategy if it's e.g. a division that's being closed.

If the company isn't in dire financial straits, it's common to give some severance pay, usually tied to length of employment. This isn't a requirement.

With the US's ridiculous system of getting most health insurance through your employer, it does mean that you lose many of those health benefits. There is a legal provision called COBRA where you can continue for a time on your previous employer's health care (at your expense). The ACA (Obamacare) makes losing your health insurance less dire than it was in the past (for things like "pre-existing conditions"), but it still sucks.