Infinite elasticity. A horizontal demand or supply curve means an infinite quantity is demanded or supplied at one specific price: quantity is extremely responsive to price, going from zero for prices close to P to infinite when the price reaches P. Perfectly elastic curves are unrealistic, but goods with readily available inputs whose production can easily expand (pizza, bread, pencils) have highly elastic supply, and luxuries with many substitutes (cruises, sports vehicles) have highly elastic demand.
Zero elasticity. A vertical curve means a percentage change in price, no matter how large, leaves quantity unchanged. Goods with a limited supply of inputs (diamond rings, apartments facing Central Park) have highly inelastic supply; necessities with no close substitutes (life-saving drugs, gasoline) have highly inelastic demand.
Switch between the two cases in the display options; the sliders move the lines.