In most cases Quand and Lorsque are interchangeable and they both mean When, but sometimes they differ in meaning:
- Quand & Lorsque - temporal correlation (Lorsque is just more formal):
Je marchais lorsque/quand tu m'as téléphoné (I was walking when you called me)
Lorsque je t'ai vu, j'avais peur (When I saw you, I was afraid)
- Quand - repeated correlation (meaning "chaque fois que"):
Quand il est là, elle ne parle pas. (Whenever he is there, she doesn't speak)
- Quand - interrogative adverb:
Quand vient il me voir?
- Lorsque means "whereas" too:
J'ai crié lorsqu'il a fallu courir (I screamed when (whereas) I should have run)
I hope this helps. Bye!