Regarding Datomic: I'm not an expert for Datomic, but AFAICS Datomic is primarily a database, while wolkenkit is an out-of-the-box full-featured framework for business logic, storage and so on.
Event sourcing itself is a fairly straightforward concept which the parent does sum up though - store events which describe what happened to an entity in a system rather than the resulting state. The state of an object can be derived via application of a left fold over the history.
[1]: https://willi.am/blog/2014/10/14/for-the-last-time-git-store... - this is the best reference I can find quickly, though the git manual would obvioiusly be more authoritative.
Conceptually, it's still capturing the deltas, instead of storing the current state and overwriting it. Storing the history is the key ingredient here, and of course, there are differences in how to achieve this.
Both are common in systems, but only the latter is event sourcing - at least if the definition is to have any use and not become overloaded.