Adware/HalfLemon is classified as Adware.
Adware are software components that display unsolicited advertisements.
The Fortinet Anti-Spyware Research Team is currently in the process of creating a detailed description for this Spyware.
For additional information about Adware, and how Fortinet classifies Spyware threats please see Fortinet's paper in PDF format:
Spyware Classification and Terms.