What is the difference between marginal utility and diminishing marginal utility?
Diminishing marginal utility is simply one "kind" of marginal utility. Marginal utility can be increasing, decreasing, or stable.
Marginal utility is simply the added benefit that a consumer gets from consuming the next unit of some good or service. In other words, if I have bought and eaten two sandwiches and I buy and eat another, the extra benefit I get from the third sandwich is my marginal utility from consuming the third sandwich.
Diminishing marginal utility comes if I get less marginal utility from consuming the next unit than I got from consuming the previous one. Using the sandwich example, if I got more marginal utility from the second sandwich than I do from the third (maybe I'm getting too full and didn't really want the whole third sandwich), my marginal utility is declining.
So, there is not really a difference between these two. It is just that diminishing marginal utility is one "kind" of marginal utility.