What is the difference between a ratio like 2:3 and a fraction 2/3?

They are closely related but used a little differently. A ratio 2:3 compares two quantities, for example 2 boys for every 3 girls. The fraction it equals depends on what you compare. The ratio of boys to girls is 2/3 because you compare 2 boys against 3 girls. But the fraction of the class that are boys is 2/5, because the total is 2 + 3 = 5 parts. So a ratio and a fraction can match, but you must be careful what the fraction is 'out of'. Ratios are about part-to-part, while fractions are usually part-to-whole. Read each question carefully to see whether they want part-to-part or part-to-whole.