Answer: Good question with straight forward answers.
A simultaneous submission means that you are taking one story and submitting it to more then one agent or publisher.
A multiple submission means that you are taking two or more stories and submitting it to the same agent or publisher.
Usually guidelines are pretty straight forward about this. Either you can or can't do it.
The rule of thumb if neither is specified: Simultaneous submissions - go ahead and submit to the target market. Make sure you include in the query or cover letter that it is a simultaneous submission. Multiple submissions - if the target market doesn't specify that sending more than one submission is okay, then don't do it.
Always make sure you follow submission guidelines carefully when submitting. It could mean the difference between a rejection and acceptance.
Peace, happiness, and submission bliss,