Investment :-  According to economics , any purchase of asset which help in production process . Example- Construction of building, Purchase of machinery ,etc

''Investment or Capital formation refers to addition to the capital stock of the economy .''

Gross Investment : Gross Investment is addition to the stock of capital before making allowance for depreciation. Ex- Ram buy machine X worth 25K ,after one year the value of X becomes 20K(repair cuts actual value) and then Ram buy machine Y worth 30K. 
So, Gross Investment = 25K + 30K = 50K 

Net Investment :  The actual addition made to the capital stock of economy in a given period is termed as net investment. 
  Net Investment = Gross Investment - Depreciation 

Depreciation : Depreciation refers to a fall in the value of fixed assets due to normal wear and tear passage of time or expected obsolescence. 

Depreciation of assets is mainly due to 3 reasons :-
  1. Normal wear and tear 
  2. Passage of time - even if fixed assets are not being put to use in business, their value falls due to natural factors like rain , wind , weather etc,. 
  3. Expected Obsolescence : Loss in value due to change in technology or change in demand for goods and services. 
Q. What is the difference between depreciation and capital loss ? 
Ans. Depreciation is due to expected obsolescence like change in technology or demand for goods and services whereas Capital loss is due to unexpected obsolescence like earthquake , floods, theft, accidents etc,