One of the main reasons for using the + time signature instead of * is that it becomes easier to count when the eighth notes are subdivided. Since there is a number on each eighth note, sixteenth notes can be counted as + (and) as demonstrated in the following example. These sixteenth notes may be played straight or swung.
Swinging 16th notes also works well in * time. Here is an example which also features a 16th note triplet in bar 2, which is three sixteenth notes over the time of an eighth note.
Another thing you may encounter in slow songs is thirty second notes. One sixteenth note divides into two thirty second notes. The following example is shown in six eight time. The thirty second notes are counted 1e+a 2e+a 3e+a, 2e+a 2e+a 3e+a etc, but are probably best felt rather than counted.