"Genuine," "OEM," and "aftermarket" get used loosely, but they mean different things and it affects both price and fitment confidence.
Genuine parts come in the manufacturer's own branded packaging (e.g. Toyota, Honda) and are typically the most expensive option, sold through dealership networks.
OEM (Original Equipment Manufacturer) parts are made by the same factory that supplies the car maker — often physically identical to the "genuine" part, just sold under the component maker's own brand (Denso, Bosch, NGK) at a lower price, since there's no dealership markup.
Aftermarket parts are made by third-party manufacturers not connected to the original supply chain. Quality varies enormously — some aftermarket brands meet or exceed OEM specs, others cut corners on materials or tolerances to hit a lower price point.
For safety-critical components — brakes, suspension, steering — sticking with OEM or genuine is the safer call. For lower-stakes items like floor mats, phone mounts, or decorative trim, a reputable aftermarket option is usually fine and considerably cheaper.