Simply , Gross Block Means - Total Value of Asset ( Its Original Puirchase price+any capex)
Net Block Means - Original Value Of Asset( As Above) - Till Date Charged Depreciation
Normally "Accumulated depreciation account " is maintained which is credited by the depreciation charged each year on the asset of the company , Once you debit current period depreciation , such depreciation shall be trasnferred to Accumulated Depreciation Account .
For Reporting purpose under companies act, it need to be made like Gross Block -Accumulated Depreciation - Net Block , Pls find example below it shall clear your query more precisely .
Eg. Asset purcahsed on 1.4.2013 , at price of Rs.20,000 Rate of depreciation as per companies act ,say 20%,Simply First FY 2013-14 Depreciation shall be of Rs. 4,000 and WDV on 31.3.14 shall be 16,000,For Next FY ie.2014-15 , WDV shall be 16,000 depreciation charged shall be Rs. 3,200 .Now How it will be reported in Block system ?
Ans. - FY 13-14 Gross Block Rs.20,000
Less- Depreciation -4,000
Net Block on 31.3.14. 16,000
FY 14-15 Gross Block Rs. 20,000
Dep.as on 1.4.14.Rs. 4,000
Add- For period FY14-15Rs. 3,200
Accumulated Depreciation - Rs. 7200
Net Block ( Rs.20,000-7,200) = Rs.,12800
It will continue until company absorb full value or not sold off .
Why Block , Suppose in above exmaple asset is Mini Laptop, If company acquire any new laptop it shall also come under this block ,which have same depreciation rate and useful life .