What is the difference between a clustered index and a non-clustered index

When data is clustered, it is rearranged to match the ordering of the index - hence the expression 'clustered index'. 

Data which has not been rearranged but is still in its original order is described as 'unclustered' and any index on such data, which imposes a specific ordering, is therefore an 'unclustered index'. 

Clearly, there can only be a single clustered index on clustered data. However, there can be multiple unclustered indices on unclustered data.