Mits is correct. To further elaborate, the Datalist functionality in Alfresco Share is basically an online table of data that users can add to and update based depending on their permissions in the site. One of the options in creating datalists is to create a To Do list where you can assign tasks to different users.
Workflow is completely different. When a user is assigned a new workflow task, an item will appear in the users "My Tasks" dashlet. Users can click on these tasks to perform different actions depending on which workflow template was used (review documents, approve documents, accept an invitation to a site). Alfresco provides some useful workflow templates out of the box, including Adhoc Workflow, Group Review and Approve, etc. These merely scratch the surface of what you're able to do with workflow in Alfresco. Based on your business process, you can create custom workflow templates to fit your business process. For example, if you have a 10 step approval process for certain documents in your system, you can create a template to control this process, the order of the approvals, who approves, and whether or not the approvals occur in parallel or serially.