Universalism is the theological doctrine that all (or nearly all) human beings will eventually be saved, and that there are "many paths" to God and heaven.
This is in contrast to Christianity, which teaches that salvation comes only through Jesus Christ, and that he alone is the only way to heaven. Those who are saved, are saved through him, regardless if they realize it or not. There is only one path to heaven, and that is through Christ. There are not "many paths," and not all people will be saved.
Even Catholic Christianity teaches this. Though Catholic Christianity acknowledges that it is possible for somebody to be saved THROUGH CHRIST even if that somebody doesn't know who Christ is. Case in point, infant mortality is one example. A baptized baby dies just a few months after birth. That child would be saved, and go to heaven, because of Christ. Christ saves the child. Christ provides the means for him to go to heaven. The child is saved THROUGH CHRIST, even though the child has no concept who Christ is. Catholicism extends this same rationale to older children and even adults, provided the ignorance of Christ is through NO FAULT of their own.
To use an analogy to better explain the Catholic Christian way of looking at salvation, lets say that a small group of men are sitting at the bottom of a deep cave. All of them are looking for a way out of the dark and twisting caverns. The men represent humanity, and the cave represents our fallen state of original sin. The path leading out, and the sunlight outside the cave represents Jesus Christ. Blinded by the total darkness of the cave, one man decides to feel his way out just using his fingers and his sense of feel alone. He represents the atheist. Another man has a jar full of fireflies, and he's able to illuminate the path just in front of him. He represents the Hindu, Shinto, and other polytheist believers. The next man has a candle. He represents the Muslim. The next man holds a flashlight. He's a Jew. The last man also has a flashlight, and a map showing the way out. He's the Christian. In this example, it is theoretically possible for all of the men to escape the cave using their own means. But it is unlikely that many will. Whoever does escape though, by whatever means he uses, will still have to follow a particular path and go toward the sunlight once he's nearly out, and that sunlight represents Jesus Christ. Nobody gets out of the cave without going toward the sunlight, and some are more likely to find their way out than others. There is only ONE PATH out of the cave - not many. All must follow this same path, and go toward the same sunlight at the end. Nobody gets out of the cave without following this same path, and stepping into the same sunlight. This is why it's so important for Christians to evangelize, because giving the gospel to your fellow man is a lot like giving a flashlight and a map to somebody trapped at the bottom of a cave.
Univeralists on the other hand, don't bother spreading the faith, because in their view, all religions lead to the same place. But they fail to see that religion is not a "path," it's a "tool," and what they're effectively saying is that a candle is just as good as a flashlight, and a flashlight is just as good as a jar full of fireflies. They're also saying it really doesn't matter if you have a map or not. To the Christian, this is nonsense! Clearly the man with the flashlight and map is far more likely to find his way out of the cave than the poor fellow just using his fingers, and everyone else in between.