I saw a lot of questions lately about it and each time, I see someone  trying hardly to answer in a couple of lines in the comment section. It's probably worth a little more time and details !  
Let's gather the information so the next time I see someone asking, I'll just send him to this blog and hopefully, you'll do the same ;)

https://s16.postimg.org/sru2rnz85/Girl_Cartoon_bts.png

Graphene is the technology behind BitShares (also behind Steem, Peerplay and Muse but let's keep our focus). 
It has been created by Dan Larimer and its team during the creation of BitShares 2.0. (Dan Larimer is DanTheMan)

BitShares is the platform running on Graphene and providing all kind of economic tools such as Decentralized Verification, Public P2P Network, Industry-Grade Security, DPOS Consensus, Decentralized Decision Making, Stakeholder-Approved Project Funding, Decentralized Exchange, Counterparty-Risk Free Trading, Integrated Payment Solution, Price-Stable Cryptocurrencies, Referral Program, Prediction Markets, Recurring & Scheduled Payments, ...

If you download the desktop wallet, you'll see the "basic" version of the BitShares graphical interface. In other words, the non-customize version.
This version is open source and can be exploited freely. Anyone who wants to build an exchange can use it and launch its own on the BTS blockchain.  The best part, you can use/share the order books of the others exchanges running on BitShares so your new exchange already has volume on the first day ... isn't it amazing ?

https://s6.postimg.org/v4bm1gym9/bitshares.png

Ronny Boesing, big boss of OpenLedger took this opportunity.  He used that free code and built its own exchange called OpenLedger at the launch of BitShares 2.0 (October 2015). As you may have seen, both platforms are very similars. Ronny is currently in the process of customising its own exchange so it meets OpenLedger criteria.

http://bitcoinist.net/wp-content/uploads/2016/02/AAEAAQAAAAAAAAZzAAAAJDcwOTU1ZDRlLTQ5NzEtNDRkYi1hNjE3LWRkM2RjNGY1YjhjYw.png

Peerplays also jump onto this occasion to build its own exchange called FreedomLedger. 

http://cryptotechlabs.io/wp-content/uploads/2015/10/FreedomLedger-Logo-tagline-trans-1-1-200x200.png

( If I wanted, I could also freely build my very own exchange called GlitterFartLedger ... )

These graphical interfaces may change over time BUT all the operations, trades, accounts, assets, ... on OpenLedger or FreedomLedger are occurring in the BitShares ecosystem. What you do in OpenLedger or FreedomLedger can be seen in the BitShares Blockchain.

I'll end up this quick article with a couple important points !

OpenLedger is not only an exchange, nor Peerplays is only freedomLedger. These two exchanges have been created by entrepreneurs with ~~big plans~~ ... huge plans !!!

Peerplay is building a new Graphene based blockchain aiming the gaming industry ... just that ! 

Ronny is building so many things around OpenLedger than I have a hard time to follow. If some of you are interested in understanding what is OpenLedger, what are OBITs and how Ronny articulate all its projects around the concept of decentralised conglomerate, I'll gladly write a quick blog on the subject. 
Drop a comment and consider it done !

I'll let you guys enjoy the rest of your Tuesday, cheers !

If you enjoyed the reading, consider following me , @GlitterFart