If I start hard mode, will I lose my world with all buildings, NPCs, and chests with all items in it?
You will not lose any buildings, NPCs, or chests. The game will, however, get harder with stronger monsters and access to tougher bosses along with more content such as better ores and other such things.
Hardmode gives new ores like Mythril, Cobalt and Adamantite and new harder bosses like The Twins, The Destroyer and Skeletron Prime. It also adds a bunch of new, harder mobs.