APDS (Armor Piercing Discarding Sabot) is a high penetration, low to medium damage round used most often in higher level matches. This shell is very accurate and it is not restricted by spaced armor making it very effective versus low HP but high armor tanks with spaced armor, however do not use it against a high HP low armor tanks or it will be very much useless.
APDS is best used against tanks with higher armor and HEAT countermeasures such as spaced armor and reactive armor. It doesn't seem to do a lot of damage in tanks with low damage stats ex. self propelled guns. This makes HEAT more popular in lower level matches.
This round is a bit more accurate at long range than all the others and can travel faster to its target.