A payline is a direction in which the same symbols will be summed up in a winning combination.
Scatter symbols give free spins; some have a separate symbol or a condition under which a player can get free spins (according to the specifics of each machine). For example, in some slot machines, the bonus game is a set of free spins. Please note that the free spins feature is not available in some slots.
Wild substitutes for any symbol on a payline that is not enough to make a winning combination; for example, Q + Q + Wild will give a winning combination of Q + Q + Q. Wild cannot substitute for the bonus game symbol and scatter.