What is the difference between classes and IDs?

Classes refer to a whole group of elements. For example all paragraphs on the website should contain red writing. In order to do this they all have to belong to the same class. We give them a class by adding <p class = "yourClassname"> to each paragraph we want to be part of the group.
An id on the other hand can only be assigned to one certain element. E.g. you want only one paragraph on the website to have blue writing. This paragraph has it's own ID by adding <p id = "yourIDname">.
If an object has a class and an ID the ID overrides the class settings. Please make sure you never give one ID to more than one object!