a love lock is a padlock which lovers attach to a bridge, gate, fence or other monument to symbolize their love. paris' pont des arts bridge is perhaps one of the most famous locations for these locks, though the practice has spread to virtually all of the 12 bridges crossing paris' seine river.
I had always thought the love lock tradition began in paris, but this is incorrect. the practice was happening earlier in asia & eastern european countries & only moved to the city of light in roughly 2008.
attaching these locks to public structures is considered vandalism in most countries & the locks are periodically removed by local governments. but you'll still find them en masse on the bridges of paris.