Sram PG-820 8-Speed Cassette 11-30T Black

SRAM Corp. is a privately owned American company that produces bicycle parts.
SRAM was founded in 1987 in Chicago, Illinois (USA). SRAM manufactures mountain bike derailleurs, gear shifters, chains and cassettes.
It also offers a range of gear adjusters, brake levers and drum brakes for regular city bikes.
In October of 2006, SRAM introduced a complete group (all moving components) for road bikes.

SRAM’s number one competition in the business is Shimano.
SRAM only has a small share of the mountain bike parts market.
In October 2006 SRAM started manufacturing parts for road bikes, like its competitors Shimano and Campagnolo.
Over the years, SRAM has taken over companies like RockShox (mountain bike suspension), Avid (mountain bike brakes) and Truvativ (bicycle parts), that now make up the SRAM Group.
This allows SRAM to offer a product range including a complete range of mountain bike parts and to better compete with Shimano, which also marketed a complete mountain bike group. 

SRAM produces groups in various price-quality classes, each with a different name.
Derailleurs and shifters are sold in sets in the same price-quality classes.
SRAM also sells shifters that are compatible with Shimano products.
These so-called trigger and grip shifters use a different system than Shimano products.
Some cyclists opt for the SRAM system, as they find it much more convenient.
SRAM chains and cassettes are compatible with Shimano either way and are sold separately from other sets of parts.
This is another reason why some cyclists will opt for SRAM products in combination with Shimano parts, as these components are lighter, better looking and/or cheaper than Shimano products.