Difference between a skiplist and an Indexable skiplist
An Indexable Skiplist is a skiplist with the added property that it can quickly lookup the $m$th entry (in order) in a skiplist in $O(\log n)$ time in a skiplist of $n$ entries.  In other words, it takes a skiplist that is not an indexable skiplist $O(n)$ time, on average, to find the $m$th element in order in a skiplist, while an ordered skiplist can do this in logarithmic time.

It does this by keeping track of the width of the links in the skiplist.