They look at side effects of a treatment – for example, does it make people sick, raise their blood pressure and so on. Phase I trials involve only a small number of people, who are often healthy volunteers.
Trials test the new treatment, usually in people who have the condition for which the treatment is to be used. This is to make sure the treatment is safe and has some effect on that condition. Phase II trials also help to find out what is the dose of treatment might be needed to be effective.
Trials involve larger numbers of patients (hundreds or sometimes thousands), who are usually randomised to receive the new treatment or the best available current treatment (or sometimes a placebo). They aim to assess how well the new treatment works, and usually take longer than Phase I or II trials to run.
Trials are done after a drug has been approved by drug regulatory authorities. They are carried out to gather information on the drug's effect in various populations and any side effects associated with long-term use.