If you dont have submenu, it doesnt matter. But globally, the class current is on the menu element of the page that you are currently watch while the class active is on every active menu. It mean that if one of its descendant has the class current, is is active.

Example

level 1
level 2
    level 2.1
    level 2.2
    level 2.3
        level 2.3.1
level 3
    level 3.1
level 4

In this tree, is you are on the page level 2.3.1, level 2.3.1 will have the class current while level 2, level 2.3 and 2.3.1 will have the class active.

level 1
level 2 .active
    level 2.1
    level 2.2
    level 2.3 .active
        level 2.3.1 .current.active
level 3
    level 3.1
level 4

It just give the state of the element.

Note

It is not jQuery that put those classes, it is joomla, this answer is based on the question posted inside your question. Those classes are put on menu element.