For those that haven't come across the term eDiscovery, or have heard it mentioned before but are unsure of what exactly it is, here's my very high level explanation:
eDiscovery refers to information or content searched for and packaged in a litigation. So for example company A is taken to court by company B and needs to gather all information / discussions / emails referring to the business deal that has gone wrong to present it in court. The process of searching for and packaging the content is eDiscovery. (in a nutshell!)
This is a feature sorely lacking in previous versions of SharePoint - however we have gotten around it historically by building custom solutions involving search etc. to get by and satisfy the legal fraternities.
eDiscovery allows for the creation of cases and within these cases to create custom queries and holds to ensure we gather all the content we need. The really impressive bit is that in SP2013 the sources of these searches can include Lync conversations, eMails, documents and other general Share|Point content as well.
To start off we create a eDiscovery center (much like we would a records or document centre).
There's more to the eDiscovery features, you can create in place holds so that even if staff change or delete items these mails/conversations/documents/etc are kept in tact and on record for presentation.
In short, well done Microsoft!