Depreciaiton is neither an asset nor a liability.
Assets are any physical property that has a value and are needed in order to run a business and depreciation are basically a reduction in the value of the assets.
During each accounting period (year, quarter, month, etc.) a portion of the cost of these assets is used up. The portion being used up is reported as Depreciation Expense on the income and expenditure(non-trading) statement or on the Profit and loss statement(trading).