There are many pattern houses (like publishing houses), but there are four large brands: Simplicity, and McCall, Butterick, and Vogue. Although McCall owns Butterick and Vogue, the patterns and styles are very different. There are many other smaller brands, like Burda, or independent designers, like Amy Butler, who produce and sell their own patterns.
Despite the differences between each company, if you can understand how to use one pattern, you should have little difficulty with the others. In this post, I won't be touching fabric or my sewing machine at all, but instead going through what you should pay attention to when you're choosing a pattern. You must choose, but choose wisely.