Becoming a Specialist is the highest level of expertise that a vet can attain in the UK. It's the equivalent to being a consultant in human medicine.
In the same way that your GP (general practitioner) doctor is your first point of call for your own medical concerns, your GP primary care vet is where your pet's treatment journey begins. Your pet's care will more than often be totally covered by your excellent GP but when more complex diagnosis or treatment may be beneficial, you should be offered the option of referral to a Specialist vet.
It takes in the order of 10,000 - 12,000 hours for a vet to become a Specialist vet. A vet wishing to become a Specialist will normally do one or two 12 month internships at a Specialist centre and then apply for a 3 year Residency where he/she trains directly under other Specialists spending 100% of their time on their area of specialism - so in total, the process generally takes about 5 years and also includes publishing clinical research papers before the vet is ready to sit the Specialist exams, which can sometimes have a pass rate of only 30%.
Candidates who pass the exams are known as Diplomates and win the hard-earned right to be called a Specialist.