THERE IS A DIFFERENCE! Let's dig in, shall we?
A diffuser oil generally tells on itself in a few ways. One of those ways is that they will have ingredients other than essential oils. You may see descriptions such as fragrance blend, fragrance, or my fav, other ingredients as needed. Another tell tale sign is the price. Diffuser oils are generally less expensive than concentrated essential oils (depending on the oils).
An essential oil will give itself away as well. If the ingredients list is longer than the name of the oil, or if the essential oil is not the first tangible ingredient (second ingredient for Body Love products, love being number 1), it is more than likely not a pure essential oil. Concentrated essential oils also have two characteristics; they are not skin friendly as a standalone product, and they actually have health benefits. Our fragrant diffuser oil pals have no health benefit. They are strictly for smell goods.
Can both oils be used in a diffuser? Of course they can.
Can they both be used on skin? Of course they can, but they both MUST be carefully diluted with a carrier product (lotion, body butter, oil, etc).