It's an activity, which is resembling or simulating science but is based on fallacious assumptions.
To understand pseudoscience it is important to understand the essentials of scientific thinking:
Scientific theories are
- based upon empirical observation
- description of the empirically found phenomenona or group of phenomena
- explaining a range of empirical phenomena, i.e the formulation - often mathematically - of an hypothesis to explain the phenomena.
- inveneting test methods to prove the phenomenon.
- confirming or falsifying the theory by empirical tests or with the discovery of new facts
- the tests have to be verifiable by other people without any preconditions based on religious or metaphysical beliefs
- the scientific approach to new ideas is based on skepticism rather than gullibility