Quantum superposition and quantum entanglement can be considered as hallmarks of quantum mechanics, crucial for the theory of quantum information. Concentrating on these basic features we demonstrate that the set of quantum states and the set of allowed operations are much larger than the corresponding sets known from the classical theory and therefore they offer us novel opportunities for quantum information processing. In particular, we review similarities and differences between classical and quantum gates, and show how they can be used to perform various computational tasks. Furthermore, we discuss briefly the effects of quantum decoherence, caused by interactions of the system analyzed with an environment,  which destroy quantum effects and bring the quantum system  back to its classical form.