Trademark vs Service Mark: Kya Baat Hai?

Maine ye dono concepts padhne se pehle hi samajh liya tha, lekin jaise-jaise padhta hoon, tohare hi maze mein padh raha hoon. Ye dono related hain, lekin kya hai difference? Kya trademark koi service mark ka case nahin ho sakta? Aur agar koi service mark hua, to usse trademark kya khaungi? Maine question 3 baar padha aur abhi bhi samajh nahi aaya. Kyun ni, trademark aur service mark dono hi intellectual property ke under aate hain. To koi difference kyun hai? Koi example dohne wala hai, toh please help. Agar koi baat main galti se likh raha hoon, to koi correction kare.