In the US "Athletics" is generally understood to be any sport that is athletic. The "Athletics Department" at a school oversees Swimming, Squash, Football, Basketball, etc. Track & Field is a specific sport for which a school would compete: while individuals win their Track or Field competitions, the school gets points for each individual competition. This may not be how the terms are viewed outside the US, though. But high schools will have "Track & Field season", usually in the spring, which is when they have their meets.