What is the difference between "Lastenheft" and "Pflichtenheft" in the V-Model?

Lastenheft is the customer's "what" (their demands); Pflichtenheft is the contractor's "how" (their refined plan to meet those demands).

Lastenheft (Customer Requirements Specification):

• Created by the customer
• Describes all demands to the contractor regarding the subject of the contract
• Documents deliveries, services, and demands of the users
• Documents constraints to the system and development process
• Describes what is made for what (the "what")

Pflichtenheft (System Requirements Specification):

• Created by the contractor
• Based on the Lastenheft
• Contains implementation suggestions that the contractor has elaborated
• Is a refinement of the Customer Requirements Specification
• Describes how requirements will be met

Analogy: Lastenheft = "I want a car that can seat 5 people and get 30 mpg" Pflichtenheft = "We will build a sedan with these specific components..."

Go deeper:

• Product requirements document (Wikipedia) — how customer "what" and contractor "how" specifications are structured and used.