Well, paradoxically, it needs to be both brief and detailed. It is necessary to mention specifics of jobs where possible, not only general terms. Yet the description should be comprehensible to customers, so they quickly understand what the invoice is for.
If your services require materials, then the invoice is an appropriate place to provide a detailed description of what and in what amounts has been purchased for the task. ID numbers of materials used are details worth mentioning. Your hourly wage should be noted here as well. If it is appropriate, you could add some photos or proofs to show what has been done, like a ‘before/after’ photo set.