An interim service is the basic maintenance check, due every 6 months or 6,000 miles, whichever comes first. It covers worn tyres, faulty lights, leaking hoses and an oil change. Coolant and brake fluid are topped up when required so the car stays roadworthy between fuller visits.
A major service should be carried out about every 15,000 to 20,000 km so the car stays in shape and the chance of a breakdown falls. Filters, plugs where accessible, and a wider under-bonnet inspection sit on top of the interim list.
A full service should be carried out every 12,000 miles or 12 months. A trained mechanic can spot damage early. It is unlikely you would see the same faults yourself, so the annual visit is the practical way to prevent a small leak becoming a larger repair.