Description: Elevation is the absolute difference between the highest summit and the lowest valley. It may also be named altitude difference or altitude range.
Ascent is being displayed as ascent/descent value which is the gain in only positive / negative altitude differences.
Example: With an altitude series of 10, 13, 15, 13, 11, 16
the elevation is 10..16 = 6
the ascent is 10..15 + 11..16 = 10
and the descent is 15..11 = 4
Tip: As most GPS receivers tend to be very inaccurate with the recoding of altitude data, the ascent and decent values can become unnaturally high as only small gains can sum up to huge values. In TrailRunner > Preferences > Advanced you can set a high pass filter for the ascent/descent calculation. See Filter altitude noise.
Additional note: It may even happen that you record a round-trip and the ascent and descent values differ. This comes from the simple fact that altitude recordings made by a GPS are extremely inaccurate. To calculate the altitude, the GPS requires good reception of at least three satellites. If reception differs during two visits of a location, the derived ascent and descent values will also differ.