"Employment at Will" and "Right to Work" are two different concepts that can be confusing and are often mixed up by employees. But they are very different concepts.
"Employment at Will" means that an employee can be terminated at any time, for any reason or for no reason at all. If the employer decides to let you go, that's the end of your job--and you have very limited legal rights to fight your termination. If you are employed "at will", your employer does not need good cause to fire you. In every state but Montana (at last check), employers are free to adopt at-will employment policies, and virtually all do. In fact, unless your employer gives some clear indication that it will only fire employees for good cause, the law presumes that you are employed at will.
Even "at will" employees, however, cannot be terminated for an illegal reason. If an employer fires someone because of his or her protected traits (race, age, disability, gender, religion, etc), or in retaliation for protected conduct, it is still illegal - even if the employment was "at will."
"Right to Work" is a very different concept that has to do with union membership. A "right-to-work" law is a state statute that prohibits union security agreements, or agreements between labor unions and employers. Generally speaking, they forbid union "closed shops". A closed shop is one in which union membership is required for employment if your job is covered by an existing collective bargaining agreement between the employer and employee. In a right to work state, a union can be elected to represent the workers but the workers cannot be required to join the union or pay union dues.
The sounds pretty good to many employees at first and Right to Work laws have gained some traction as a result. Right-to-work laws exist in 24 U.S. states, mostly in the southern and western United States, but also including, as of 2012, the midwestern states of Michigan and Indiana. The downside of such laws is that they dramatically weaken unions buy effectively starving them of the funds they need to operate and organize. As a result, union membership in right-to-work states has dropped dramatically. Lower union membership has in turn led to a drop in workers's wages and severe damage to job protections for workers in those states.
Not surprisingly, right-to-work laws have been strongly championed by anti-worker political action groups, such as U.S. Chamber of Commerce. Such groups have spent millions on running misleading advertising and purchasing politicians who will support their efforts to curtail workers' rights and suppress wages.