Just like other languages are strict by default, but can be made lazy via sequences, streams, generators,...
It is just a matter which defaults might be better for a given application.
Feel free to point if I've got any of the above wrong.
Update: Thanks for the replies, people.
In Haskell, the primary mechanism for inducing evaluation is the case expression (if/then/else and pattern matching are syntactic sugar for case).
Thunks (lazy expressions) in Haskell are created with let and then forced with case. It's very straightforward at the most basic level, there's just a lot of abstraction built on top of it that can obscure what's going on.