Lisp is a very different beast from what you used to know. According to Paul Graham, there are nine ideas that make Lisp different (these ideas have existed since the late 1950s), and they are:
Conditionals (remember, we are talking 1950s–1960s)
Functions as first-class citizens
Recursion
Dynamic typing
Garbage collection
Programs as sequences of expressions
The symbol type
Lisp's syntax
The whole language is there all the time: at compilation, runtime—always!
Note
If you can, read Paul Graham's essay Revenge of the Nerds (http://www.paulgraham.com/icad.html), where he talks about Lisp, what makes it different, and why the language is important.
These ideas have thrived even after the Lisp age; most of them are common nowadays (can you imagine a language without conditionals?). But the last couple of ideas are what makes us Lisp lovers love the syntax (we will fully understand what they mean through this chapter).
Common languages are trying to achieve the very same things now...