This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Entity Cache is a mecanism implemented by the Application Servers in order to increase the applications performance. Instead of loading data from the database every time you need it, when loading data for the first time this data will also be put in the cache, then the next time you need it, the data is available from the cache and no read from the database is needed. How is this Entity Cache working, how can be used and configured, is specific to each Application Server so if you want to use it you have to look for documentation for your app server. Or you can post here what app server you are using, and there might be some guys that are able to help you.