An operad is an algebraic device which encodes a type of algebra. The classical types of algebras, ie. associative, commutative and Lie algebras give the first examples of algebraic operads. Operadic point of view has several advantages. Firstly, many results known for classical algebras, when written out in operadic language, can be applied to other types of algebras. Secondly, operadic language simplifies the statements and proofs. Thirdly, even for classical cases, operad theory has provided new results. We start with several equivalent definitions, together with examples, and few basic properties.