A clinical study involves research using human volunteers (also called participants) that is intended to add to medical knowledge. There are two main types of clinical studies: clinical trials and observational studies.
In a clinical trial (also called an interventional study), participants receive specific interventions according to the research plan or protocol created by the investigators. These interventions may be medical products, such as drugs or devices; procedures; or changes to participants' behaviour, for example, diet.
Clinical trials may compare a new medical approach to a standard one that is already available or to a placebo that contains no active ingredients or to no intervention. Some clinical trials compare interventions that are already available to each other. When a new product or approach is being studied, it is not usually known whether it will be helpful, harmful, or no different than available alternatives (including no intervention). The investigators try to determine the safety and efficacy of the intervention by measuring certain outcomes in the participants. For example, investigators may give a drug or treatment to participants who have high blood pressure to see whether their blood pressure decreases.
In an observational study, investigators assess health outcomes in groups of participants according to a protocol or research plan. Participants may receive interventions, which can include medical products, such as drugs or devices, or procedures as part of their routine medical care, but participants are not assigned to specific interventions by the investigator (as in a clinical trial).
Doctors and other healthcare professionals need evidence from clinical trials to know which treatments work best. Without this evidence, there is a risk that people could be given treatments that have no advantage, waste NHS resources, and might even be harmful.
Clinical trials help to find out if:
Many NHS treatments have been tested in clinical trials.
The evidence for some existing treatments is incomplete, and so clinical trials are used to provide healthcare professionals with a scientific evidence base for ongoing use of these treatments.
A commercial trial is a trial that is funded by a pharmaceutical, biotechnology or medical device company. The trials are vital in the development of new treatments, which can then be taken to market. Commercial trials commonly take place on a much larger scale than academic studies, often being conducted in multiple countries around the world simultaneously. These trials are sometime run by Contract Research Organisations (CROs), such as Quintiles, who provide the logistics to allow these trials to be run.
Academic (non-commercial) clinical trials are trials or studies that are not funded by pharmaceutical or biotechnology companies for commercial ends. They are usually led by a clinician or researcher working at a hospital or university and are funded by charities (e.g. Cancer Research UK), national research funding bodies (e.g. the National Institute for Health Research), or the universities or hospitals themselves.
The UKCRC defines a clinical trials unit as a unit with overall responsibility for the design, recruitment, data management, publicity and analysis of multi-centred randomised controlled trials or other well-designed studies, working in any disease/topic area.
In simple terms, clinical trials units (CTU) are specialised research units which help clinical investigators to design, run, analyse and publish clinical trials and other studies. Trials unit provide the infrastructure and expertise to ensure that trials and studies are scientifically well designed, practical and ethical. This ensures that patients are not asked to take part in trials that are not in the best interests of the NHS, the scientific community, and most importantly themselves.
All studies and trials in the UK require approval from an ethics panel before the research is allowed to be conducted. For more information on the UK research ethics system, visit the Health Research Authority website.
 Original text from NHS website (http://www.nhs.uk/Conditions/Clinical-trials/Pages/Definition.aspx)
 Original text from US National Institute for Health website (https://www.clinicaltrials.gov/ct2/about-studies/learn)