A water-resistant bag slows water down. It is usually made from a coated or tightly woven fabric, often with a water-repellent (DWR) finish that makes water bead up, and it has sewn seams and ordinary or covered zips. It handles a light shower, but in heavy rain water gets in through the stitch holes, the zips and eventually the fabric itself.
A waterproof bag keeps water out completely, within the limits of its rating. It uses a fabric with a continuous waterproof layer, such as PVC tarpaulin or TPU-laminated fabric, joins it with welded or fully sealed seams, and closes with a roll-top, a waterproof zip or an airtight zip.
"Water-repellent" describes a surface finish, not a construction. It helps a fabric shed water, but on its own it never makes a bag waterproof, and it wears off with use and washing.
The clearest way to compare bags is the IPX rating. IPX4 means splash-resistant, IPX5 and IPX6 mean protected against water jets and heavy rain, and IPX7 and IPX8 mean protected against immersion. At our factories, IPX5 and IPX6 bags pass a heavy rain and water-jet test, and IPX7 and IPX8 bags pass a full water-immersion test of 2+ hours. Our article on waterproof vs water-resistant laptop bags shows the difference on a real product.