A garment is a particular article of clothing (especially in a religious context), or refers to the industry of clothing.
"They tore their garments (Genesis 44:13)."
"I'm going downtown to the garment district to buy a truckload of fabric."
Garb refers to a style of clothing, especially something distinctive.
"They were dressed in the garb of elves."
Apparel also means clothing, but has an elegant implication.
"I shop at Nordstrom's for the latest in fashionable apparel."