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If you planning to learn Adobe CQ (Formally known as DAY CQ) or WEM, You need following requisite.

1) Apache Sling (How resource resolution works, Eventing)
2) Apache Jackrabbit (Repository, Search, Versioning, Observation, User/Group/ACL management, Clustering)
3) Apache Felix and OSGI (Dynamic package deployment)
4) EXTJS (JS library in WEM / CQ, Though they are overriden in CQ)
5) And then finally CQ/WEM documentation (Which include CQ and CRX)

Some more useful link

1) Why OSGI
2) JCR Query (Good example of how to use Xpath queries in JCR)
3) Resource Resolution in Sling (Important to understand how resources are resolved in Sling)
4) Some examples of sling (Some live examples of sling)
5) What you can do using sling + REST (A very good presentation of what you can do using REST framework)

<More to come>