We discuss main formalisations of information and knowledge available in literature. Knowledge is considered as a meaning of data for an actor who can understand and use it. The actor can be a computer, a robot, a human being or even a virus. We make an assumption that the usage of knowledge depends on material data carrier only as much as the latter allows one to perform operations and restricts knowledge processing through capacity and time. This permits one to consider knowledge abstractly. Knowledge handling requires some tools that are called knowledge systems. These systems have different forms and they can be connected in various ways as it follows from the analysed publications. A metrics of knowledge is discussed that reflects the capability of solving problems.