I will assume UCCE here:
Call type - think of it as a call tag for reporting purposes. As call traverses through a script, i.e. different options, etc it may be tagged with different call type along the way. CT is also used to deliver a call to specific script, i.e. DN points to CT, which then is scheduled to point to script.
Skill - Self explanatory, skills are used to define queues, by assigning an agent to a specific skill that is queued to in the script, the agent is capable of handling the call.
