What is ganache, and how it is used in truffles? Does it differ from chocopaste?
Ganache is a mixture of cream and chocolate, made by heating the cream and mixing in the chocolate until smooth and fully incorporated.  Butter is then often added to give the final product a characteristic "shine".  Ganache can be used as a filling or as a coating/topping in truffles and is a frequent component in other desserts.

I've encountered "chocopaste" at an Indian grocer before, and it is essentially a homogenized mixture of chocolate and milk with other ingredients.  It has a similarity to ganache in that it contains chocolate and dairy combined.  The commercial chocopaste I've seen have also had other ingredients (added fats and/or sugars, stabilizers, preservatives, etc.), unlike ganache made at home which would contain only cream and chocolate.  In this way chocopaste is somewhat like Nutella, the chocolate-hazelnut spread.  In fact, the chocopastes I've seen came in similarly-shaped jars.

Chocopaste appears to be used as everything from a dessert topping or filling to a base for making chocolate drinks.

Here is an example of chocopaste, containing the following ingredients:


Sugar
Hydrogenated Vegetable Fat
Alkalised Cocoa Powder
Milk Powder
Permitted emulsifiers
Added flavors


In terms of substituting one for the other, the added stabilizers, sugars, and flavors in chocopaste may affect the final product.  Hydrogenated vegetable fat isn't the same as butter either, so mouthfeel and flavor will not be the same.  

I would expect different chocopastes to potentially vary in terms of flavor and sweetness.  For uses as filling/topping/etc, I would just try out whatever products interest you and compare for yourself.