Sling events helps to manage events within your appication. it is used to trigger workflows, business process like sending an email/ managing assets etc..
where as JCR obeservations are purely related to the repository and to capture events like node/properties added/deleted/moved.
You can check here for more details [1][2]
[1] http://www.day.com/specs/jcr/2.0/12_Observation.html
[2] http://sling.apache.org/documentation/tutorials-how-tos/how-to-manage-events-in-sling.html
Thanks,
Loki