"Serial" is a very general term for a bus which sends data serially on a single wire. It's similar to the word "vehicle". Just like there are hundreds of different vehicles - cars, trucks, motorcycles, buses, boats, planes, helicopters - there are also many different serial protocols - USB, RS232, RS422, RS485, UART, SPI, I2C, etc.
I think the reason you're confused is because "USB to Serial" is a stupid name for that product. It's not precise at all and doesn't tell you what it's actually doing. Chances are it's a USB to RS232 or USB to UART converter, simply converting between two different serial protocols, but you'd have to check the details to be sure.