Workbench requests hold the changes made in repository changes while Customizing requests hold the changes made in customizing table entries

Workbench request carries whatever development we are doing like creation of infoobjects and DSOs etc.
On the contraty Customizing request carries all the customizations we are doing in the system for example adding a new currency to the currency table, defining factory calendar etc.

The Transport Organiser maintains Change Requests.These requests record the changes made to the repository and customizing objects.Based on that objects changed they are
1)WorkBench Request & 2) Customizing Request.

Workbench Requests are those that involve changes to cross-client Customising and Repository Objects. The objects are independent of the client.Hence the requests are used for transferring and transporting changed Repository objects and changed system settings from cross-client tables.

Customizing Requests involve changes recorded to client-specific Customizing objects .These client specific requests are used for copying and transporting changed system settings from client-specific tables.

Refer this links http://www.allinterview.com/showanswers/57910.html
https://www.sdn.sap.com/irj/scn/thread?messageID=1816341
https://www.sdn.sap.com/irj/scn/thread?messageID=226306
and many more threads available on this.

On a more general note, Workbench requests are client independent, i.e., the changes made will be reflected across all clients.
Customizing requests are client specific.