I took karate classes as a kid, but I don't know much about it.
Well Karate was introduced to Japan in the early 20th century.
Oh, I thought it would be earlier, IDK why.  
Before being introduced to Japan, Karate originated in the Ryuku Kingdom which was annexed by Japan in 1879.
Ahh, gotcha. Are there different types of practices of karate?  Or is it all basically the same?
There are many styles of karate. Karate hit peak popularity when martial arts movies were made in the 1960s and 1970s. 
I wonder if that's when karate classes in the United States started too?
I would say it has a huge correlation to its popularity around the world at that time. People were looking to practice karate seeking a deeper study of the art and just casual interest.