Taverna has an implicit iteration system, which means that you do not normally need to worry about handling iterations. If you provide Taverna with a collection of data, the workflow will iterate over each item automatically. If, however, one of your services combines two or more inputs, you need to specify how Taverna should combine these inputs in the event of iterating over multiple values.
You can specify a cross product iteration, which combines everything in one list with everything in the other (all against all), or you can specify a dot product iteration, which combines item 1 of list 1 with item 1 of list 2, and so on (row against row). The default behaviour is to combine lists using a cross product. Detailed information is available in the list handling section of the user manual.