The main difference is the purpose for which ETL tool and OLAP tools are used.
ETL tools is meant for extracting the data from one or more than one source and loading into the Target tables.Before loading into the target it involves a lot of transformations to implement the business logics and standards.Also we have the staging area to clean the data and then load.Informatica and Datastage are few of the popular ETL tools
You can check about the popular ETL tools
OLAP tools are mainly meant for reporting purpose. It loads the data extracted and loaded by ETL from the target tables to the OLAP repository and then makes the necessary changes for creating a report out of it.These reports are viewed by the End user and they can check their Datawarehouse.Cognos and OBIEE are few of the powerful reporting tools
You can check about the Popular reporting Tools