What is the difference between ethernet and serial?

Most likely (since the context of the use of terms is not specified) serial is RS-232.
Ethernet is a network interface.

They have a different purpose: RS-232 is used for communication with peripheral devices (for example, a modem), and Ethernet is used for connecting to local networks.

RS-232 ports typically transmit individual bytes as they are received. Most Ethernet devices, by contrast, will wait until the packet is fully provided before starting transmission, and will wait for the entire packet to be received and validated before making it available to the software. And although bits and bytes can be sent sequentially, software doesn't care much about it. It just knows that a short period of time after the controller has been when a packet is "fed" and told to send it, the other controller will tell you that the packet is available and allow the program to subtract it.

The serial port guarantees the transfer of bytes of data provided by the software, in chronological order, and also in the opposite direction. All Ethernet systems ensure that when sending a packet from one node to another, the location of the bytes provided to the receiver will match the location of the bytes provided by the sender. However, there is no general specification requiring that the chronological order in which data is transmitted has anything to do with its location in the packet.

In such cases, in order to facilitate understanding and not to confuse the terminology, it is worth making analogies.

The analog of the Serial port (aka rs232, COM port) will be the USB port, while the analog of the Ethernet will be WI-Fi. In this case, I focus on the main function of the two concepts.