Depletion counters are used by two very different cycles of lands to limit the activation of their better abilities.
The term depletion land may be used to refer to either or both of these cycles, but may also refer to mechanically similar lands that use other counters.
The first cycle, from Ice Age, includes the cards Land Cap, Veldt, River Delta, Lava Tubes, and Timberline Ridge. These lands are dual lands and enter the battlefield untapped, but use depletion counters to delay their untapping.
The second cycle, in Mercadian Masques, includes Remote Farm, Saprazzan Skerry, Peat Bog, Sandstone Needle, and Hickory Woodlot. This cycle enters the battlefield tapped and with two depletion counters, and has only one mana ability, which removes a depletion counter to add two mana, rather than the typical one. These lands are sacrificed when their mana ability resolves if they have no further depletion counters.
Two Scourge cards, Decree of Silence and Force Bubble, use depletion counters for flavor reasons, but are unrelated to the use of depletion counters as a larger mechanic on other cards.