usage of the word "and"?

It depends on the style in which you are writing. The comma before "and" in a series is called the Oxford comma. Everyone has their own opinions about the use of this comma. Sometimes leaving it off can cause confusion because the lack of a comma can sometimes be used to signal an appositive (that is, it is used to signal that the words after a comma are meant as an alternate description of the word immediately preceding the comma). For example, "I saw the couple down the street, a dog and a cat," would imply that the couple down the street is a dog and a cat, while "I saw the couple down the street, a dog, and a cat," would mean that those three were separate parties. Even styles that usually omit the Oxford comma will include it in cases where leaving it out would cause confusion. In this case, its omission is unlikely to cause confusion. If you're using one particular style, you should look up the rules for that writing style. If the job description mentions a particular style, you may want to use that style as a guide. However, if you're not using a particular style, then you can choose between using the Oxford comma and leaving it out. Be consistent with your choice throughout your resume and cover letter. If you have appositives in your resume or cover letter, you may wish to adopt the Oxford comma merely for consistency's sake.