What's the difference?

The third dose of the Covid-19 vaccine is being offered to immunocompromised individuals to improve their immune response as part of the primary course of treatment. The booster vaccine is being offered to a wider set of people who are at greater risk of being seriously affected by Covid-19 to maintain their level of protection for longer.
Booster Covid-19 vaccine

The Government is rolling out booster vaccines in order to maintain protection for individuals against the virus for a longer period of time, particularly over the winter months. Boosters are being offered to people who fit the below criteria at least six months after their last vaccine.

Booster vaccines are being given to people who:

are aged 50 and overlive and work in care homesare frontline health and social care workersare aged 16 and over with a health condition that puts them at high risk of getting seriously ill from Covid-19are aged 16 and over who are a main carer for someone at high risk from Covid-19are aged 16 and over who live with someone who is more likely to get infections (such as someone who has HIV, has had a transplant or is having certain treatments for cancer, lupus or rheumatoid arthritis)
The booster vaccine being rolled out in the UK is mainly Pfizer and Moderna, therefore may be different to the vaccines you received previously.

As of Monday 8 November, the National Booking System will allow someone to pre-book their booster appointment a month before they are eligible, making it easier and more convenient to get a vaccine.

See the NHS website for information on how to book your booster vaccine.
Third Covid-19 vaccine

From September 2021, the government has been offering a third dose of the Covid-19 vaccine for people who are immunocompromised, aged 12 and above. This is because these individuals are likely to have had less of an immune response to the first two doses of the vaccine compared to the rest of the population. The third dose is offered at least eight weeks after the second dose (but can be longer if you were vaccinated early) and is considered to be a part of the first course of vaccination.

The third dose of the vaccine is different to the booster vaccine. Those who are eligible for the third dose are likely to be offered a booster at a later date.

Those who are eligible for the third dose include:

Individuals with primary or acquired immunodeficiency states at the time of vaccination due to conditionsIndividuals on immunosuppressive or immunomodulating therapy at the time of vaccinationIndividuals with chronic immune-mediated inflammatory disease who were receiving or had received immunosuppressive therapy prior to vaccination including:Individuals who had received high-dose steroids (equivalent to >40mg prednisolone per day for more than a week) for any reason in the month before vaccination.