Well lets go over the meaning of Service Page and Landing page.
- Service Page - A Page that holds specific information regarding "Services".
- Landing Page - Any page that a user "landed" on, the user landed on this page because he clicked on a link or entered the www address.
Your service page can be a landing page for organic keywords.... I hope you understand now how different the two terms are, feel free to keep on asking questions, ill do my best to answer you 
