ASCII and Unicode are two character encodings. They are standards on how to represent difference characters in binary so that they can be written, stored, transmitted, and read from.
The main difference between the two is in the way they encode the character and the number of bits that they use for each.
In Unicode, the first eight bits matched that of the most popular ASCII page, meaning that if you open an ASCII encoded file with Unicode, you still get the correct characters encoded in the file. This helps with the backwards compatibility.