Let us discus about Group span first.

for Example,
In List Report when you have product line, product type & product name as a query item. You've product line & product type grouped, Ok?
Now you want to span product line as per product type(i.e. you want product line to repeat whenever new product type is listed beside it). You can click on product line & change the Goup span property to product type. This gives you the proper result.
Now you want to span product line as per product name(i.e. you want product line to repeat whenever new product name is listed beside it). In this case you have to first group the product name query item then & then you can change Group span property of the product line to product name. Because Group span property lists the query items which are grouped in the list.

Lets discus about Level span now.

for Example,
In CrossTab Report when you have product line as parent node in row, product type as child node of product line & product name as child node of product type.
Now you want to span product line as per product type(i.e. you want product line to repeat whenever new product type is listed beside it). You dont have Group span property in CrossTab, Ok? Here you can do some trick which is called Level span. You can add one more product type as child to product type & then cut product line from CrossTab. Then select Parent product type & change Source Type property to Data Item Value & Data Item Value property to product line. Now run the report you can see the product line level repeats whenever new product type is listed beside it. This is called a Level Span.

Remember one more thing you can not do same kind of Level Span using dimensional model. You can do Level Span using relational model only...