Straight line method means charging depreciation on the
original cost of goods and written down method means
charging depreciation on balance amount every year.

Normally the amount written off every year is the same in
Straight line depreciation method and it differs in Written
down method as the depreciated amount is calculated on the
written down value.

In S L M depreciation is evenly charged over fixed 
assets.But in W D V method depreciation is charged unevenly 
over the fixed assets.

In straight line method  the amount to be depreciated remains constant where as In written down method the amount to be depreciated decreases by each passing year.