This is one of those questions that are not as difficult to answer as you might think. The Drug Enforcement Agency (or DEA) still has marijuana classified as a schedule 1 drug. While the schedule system is, admittedly, complicated to navigate, schedule 1 means that it's not even legal to prescribe marijuana as a medication and the DEA views it as having no medicinal value. While this is clearly arguable and very much so against the current cultural knowledge and understanding of the drug, it is still how federal law handles marijuana.
In other words: pot is not legal. It's not complicated.
However, the legal status of marijuana has not prevented people from partaking of it in the past and seems unlikely to in the future.
Also, the federal government is unlikely to track down a lone person who's smoking a single joint. Most of the way the federal government controls marijuana use is big drug busts on distributors (farms and dispensaries), as well as leaning on banks to keep them from allowing bank accounts for dispensaries and other cannabis-related industries. (This is, by the way, why sometimes dispensaries can only take cash. If the credit card company or bank won't work with them on processing credit cards because they're selling cannabis products, it's not a sign that the business practice is shady, it's a problem they run into from time to time.)
As far as from state to state goes, that's really varied at this point. There's no united front across the states on legality. Some states have decided that it is 100% fine to use recreationally, in some states you need a medical card in order to buy marijuana, and of course, it's still illegal at the state level in others. In the past few years, more and more red states have determined that it's okay for medical purposes, and it seems likely that it's going to be broadly legal at the state level long before it's legal at the federal level.
According to Gallup, 66% of Americans believe that marijuana should be legalized. It's becoming more and more accepted on a broad level to talk about this, instead of being a sort of fringe belief, so it seems likely that access will grow. But there are still some confusing legality issues, and those legality issues may only become more byzantine as time goes on. But right now, even though it may be marginally legal in your state, using it could still be legally risky.
For more information about cannabis law, contact a cannabis law firm.