An example: A name brand manufacturer that produces jars of peanut butter may also produce bags of private label peanuts.
Retailers prefer to sell their store brand products because private label profit margins are generally higher. Stores are able to control the retail price without having to worry about product discounts, deals, and coupons (which eat away at a store's bottom line). Generics can save you up to 50% off and sometimes more. Look at your shirt. Does it have a guy on a horse logo? If so, you most likely paid 300-400% more to have it on there (so take good care of it).
There are times when the quality of a generic product doesn't stand up to its brand name counterpart. In general, you don't know the quality of an item until after you make the purchase.
Some products are worth the extra cost. There is one item in our household that will never be replaced by it's generic cousin:
What are some irreplaceable brand name items in your home?
What are some generic products that are saving you some mega cash?