Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
Trojan.Adservice is the detection for a malicious DLL file called AdService.dll. Once present on an affected system, it is capable of stealing login credentials, login information, and password from Web services, such as Facebook and Twitter.
Malwarebytes can remove Trojan.Adservice without further user interaction. However, it is recommended to change all passwords that could have been stolen from the affected system.