At first glance they are similar because both challenge the objectivity of knowledge, although they do it in different ways.
There are different kinds of skeptics, but the most popular kind is the one that doubts whether knowledge is possible. Some skeptics claim we cannot have knowledge, while others claim that we do have knowledge but we can never know when we do. But we need to be careful. The good skeptic will simply just doubt whether we have knowledge, but will not go the extra step and say “We have no knowledge” because that claim is itself a knowledge-claim.
Like skepticism, there are different forms of relativism. The classic relativist would say that there is no such thing as universal truth. Some relativists would say, though, that it is possible for one person to have truth—and this a classic skeptic would deny. Relativists do not simply deny there is knowledge. Rather, the position is that knowledge is relative to something, usually relative to the individual (others say relative to a culture, or even relative to our species). So, a relativist can still talk about truth depending on where he wants to draw the line, but this truth is never universal for all. Relativism nevertheless faces a contradiction. If we say “Knowledge is relative to the individual” then that statement applies to all individuals. Therefore, extreme subjective relativism contains a contradiction. It is unclear to me whether all forms of relativism fall prey to this trap.
Relativism oddly enough relates to skepticism and its counterpart (people sometimes if not always have knowledge of the world around them) in exactly the same way.
The proposition that people do not have knowledge is both true and false in relativism. The truth value of the aforementioned skeptical proposition is to be determined by the scope to which this proposition applies to.
If the proposition is made about local common sense notions such as do people have knowledge of their birth dates the proposition is false. However, if the proposition relates to global why questions such as why is there something rather than nothing, then the proposition is true.
1. Everything cannot be true, because if everything was true then a statement saying that everything is true is false would be true
2. The idea that you cannot know truth has no solid foundation, because you cannot know that you cannot know truth if you cannot know truth.
3. If the law of non-contradiction is false then it could also be true because the law of non-contradiction cannot be used to prove the law of non-contradiction untrue if the law of non-contradiction is false.
Relativism is the philosophy of all morality being relativistic and not absolute. This is based on noting that different cultures have different moralities. So we can say “That may be true for you but false for me,” or my own favorite: “One man’s freedom fighter is another’s terrorist.”