Pseudo means false or counterfeit, appearing to be genuine.
Pseudoscience means an activity resembling science or claiming to be scientific, but not founded in accepted scientific methodology.
To be accepted as valid science, an activity must follow the scientific method. This is an objective, reliable process which has been agreed upon by scholars from a huge range of disciplines and proven over centuries.
Any activity which falsely gives the appearance of genuine, reputable science can be considered pseudo-scientific. Often great effort is put into the facade and there may be no easy way to judge authenticity.
Common characteristics of pseudoscience include:
The term pseudoscience is a little bit subjective. Strictly speaking it includes scientific endeavours which contain flaws. However it is usually used to describe science which is more than flawed, in particular, it tends to imply deliberate negligence or deception. If an experiment conducted openly with invited peer review turns out to contain errors, it will probably be labeled no worse than "flawed". If the same experiment was conducted in secret with the experimenter refusing to allow scrutiny, it might be labeled more harshly as pseudoscience.
Flawed science is quite pervasive. From mainstream science communities to the fringes of unorthodox subcultures, erroneous scientific conclusions are a major problem. Scientists and science commentators must constantly be on guard against such mistakes.
It should be noted that although flawed science is common, the scientific method itself is not in dispute. It's like mathematics - every day people add up numbers incorrectly but no one doubts the process of addition. Likewise, science and the scientific method should not be held responsible when they are misused. Human error alone is the culprit.
In mainstream science, there are systems in place to weed out errors. Chief amongst these systems is peer review. This means that new theories, papers and data must be reviewed objectively by qualified professionals before gaining wider acceptance. Prestigious publications such as Nature and Science place a great deal of importance on this process.
Peer review is a fundamental requirement for a good reason - it is the best way to verify that the scientific process has been adhered to. Note that peer does not mean one's friends and colleagues - the most convincing form of peer review involves people who have no relationship to each other.
A classic symptom of pseudoscience is lack of peer review.
Note: It is fair to say that peer review is not perfect. Radical theories can sometimes struggle to find respected authorities who are willing to endorse them. However, the worst case scenario is generally that the theory takes longer to gain peer approval. If the person proposing the theory can back it up with evidence then it's only a matter of time and patience before a fair-minded reviewer can be found.