Basic Ideas

Basic Idea of Verification:

Statements that verify that specified conditions are true are conditionally compiled into the code, allowing error checking that can be turned off completely for fast execution.

Basic Idea of Design by Contract:

Routines satisfy a contract when they are called - input requirements are verified upon entry and output guarantees are verified prior to exit.

These are very simple, but very powerful ideas. Unfortunately, the main proponents of these ideas (Eiffel and C++) use bad nomenclature.

Here's a translation table, so you'll recognize these ideas in other venues:

Eiffel or C++	English
assert	verify
precondition	requirement
postcondition	guarantee
class invariant	valid state
require	verify (on routine entry)
ensure	verify (on routine exit)