The main difference between the two is the difference between the two tenses that are used. When you say “says,” this means that you are referring to an individual who is saying something in the present tense. When you say “Simon says croak.” This means that Simon is telling you to croak. When you say, “Simon said croak.” This means that Simon already asked you to croak earlier.
Basically, they mean the same thing but the tenses are different. You would use the right word depending on the tense of your whole sentence. Some people may become confused with the tenses from time to time but as long as you would study English more, it will not be too complicated anymore.