What is the Difference between accumulated depreciation and depreciation expenses?

What is the Difference between accumulated depreciation and depreciation expenses?
Accumulated depreciation is all of the depreciation ever 'accumulated' against the assets currently in service. It is shown on the balance sheet as a 'contra' (negative) asset, directly below the assets it relates to.

Depreciation expense is the current period's depreciation of the assets currently in service. It is shown on the income (P&L) statement as an expense.

Example:
Business purchased a truck for $20,000 which will last 5 years. For simplicity, we'll use 'straight-line' depreciation.

End of Year One:
Depreciation expense on Income Statement
$4,000 (1/5th of $20,000)

Accumulated Depreciation on balance sheet: $4,000

End of Year Two:
Depreciation expense on Income Statement
$4,000

Accumulated Depreciation on balance sheet: $8,000 (both years)

End of Year Three:
Depreciation expense on Income Statement
$4,000

Accumulated Depreciation on balance sheet: $12,000 (all three years)