USB to serial adapter
Yes, USB sends data serially.  USB has one data signal carried differentially on two wires.

However, when people talk about "serial" communication in some contexts they don't mean the general serial as you are interpreting it, but a "serial port" also known as a "COM" port on PCs.  Note that "COM" stands for "communication", so technically applies to all data links .  "COM port" or "serial port" or sometimes just "serial" refer to RS-232 from context.  Yes, this is all a bit sloppy, but it is common.

In your case "USB to serial adapter" almost certainly means something that presents like a normal device on the USB side but does UART-style communication on the other.  There is yet another ambiguity in whether that UART-style communication is real RS-232 or using normal digital logic levels.  The digital logic level signals are normally inverted from the true RS-232 signals.  The microcontroller does "serial" via digital logic signals, but PC COM ports do real RS-232.