Like a ground ball, the basic definition of a fly ball seems pretty self-evident: it is a batted ball that is hit into the air.
A standard fly ball, or “fly,” is usually hit high and travels some distance laterally, usually to the outfield. On the defensive side, fielding a fly ball is usually considered to be a routine play, and catching a fly ball before it hits the ground results in an out. A pop fly, sometimes called a “pop up,” is a batted ball that typically soars even higher than a routine fly ball, though it does not travel very far laterally, if at all.
Meanwhile, a line drive is a ball that, while hit in the air, does not quite typically as a fly ball. Line drives, or “liners,” are hit lower to the ground than routine fly balls, and usually with more speed and power. They have little to no arc in their flight paths and are generally more difficult to field than routine fly balls. A big reason that the third base position is referred to as “the hot spot” is because a lot of line drives get hit towards third base, often at dangerously high speeds.
