In the IELTS speaking test, candidates receive a band score from 1 to 9 in the following four criteria:
1) Fluency and cohesion
2) Lexical resource (i.e. vocabulary)
3) Grammatical range and accuracy
Scores are assigned in each criteria based on “band descriptors” (download a copy of the band descriptors [public version] from the IELTS website here).
The final band score will be the average score across the four criteria rounded down to the nearest half band score.
Examiners are rigorously trained, certified and monitored to accurately assign band scores. They apply a similar, yet more detailed, set of descriptors to the ones in the public version.
The quickest and most reliable way to improve your IELTS speaking test performance is to practice with a professional IELTS teacher who can identify your strengths and weaknesses according to the band descriptors, and then help you improve on them.