Scriptlets are just JSP files that are composed using regular Java code instead of taglib code. Note that one doesn't exclude the other, so you can even use a mix of the two.
Choosing between scriptlet code and taglib code is usually just a matter of preference, and a slight mix is not uncommon.
At the start of a scriptlet, you will typically find some kind of CmsJspActionElement and a CmsObject. These two (the CmsObject in particular) are often the central corner stones from which you can access many of OpenCms' features.