The difference is in how they relate to the beat.

Here is a crotchet (quarter note) beat:

1 2 3 4

8th note triplets fit in three notes per beat:

1 trip-let 2 trip-let 3 trip-let 4 trip-let

16th note triplets fit in 6 notes per beat, double the the 8th:

1 trip-let & trip-let 2 trip-let & trip-let 3 trip-let & trip-let 4 trip-let & trip-let

The tempo and beat remains the same, the subdivisions are different.

If you take both in isolation, they would sound the same. However, insert the tempo and the beat, and the 16th note triplet is twice as fast, with twice as many notes fit into the same space, as the 8th note triplet. It's all about subdivisions.