Many devices use nonstandard connectors for serial ports. RJ-45 is probably the most common connector used for RS-232 serial after DB-9, but unlike with DB-9, there aren't even de facto standards for the pinout. I'm aware of 4 different RJ-45 RS-232 pinouts, and there are probably others I haven't seen yet.
None of this means that people are somehow converting Ethernet to serial. They merely happen to use the same connector.
There are many products that do provide that conversion, and in fact most of them do use the RJ-45 connector for their serial side. For an example of a single-port converter, there's the Digi One SP. More common are boxes that provide multiple serial ports, like the Digi PortServer and the Avocent (neé Cyclades) Console Servers. These are just two examples out of many. Digi and Avocent are easily the two biggest players, but there are lots of smaller companies doing things like this.
Some of these boxes present themselves to the OS as
/dev/ttyWHATEVER by installing a driver. These have the advantage that any program that knows how to talk to a serial port can talk to the remote device plugged into the converter. For the most part, the driver makes the converter appear no different from a local serial port. For example, if a program opens one of the converter's
/dev/ nodes and calls
cfsetospeed() on it to set the serial port's bit rate, the driver forwards the command to the remote converter box, which changes the serial bit rate on that port.
The main problem you run into with that type of converter is that it isn't always possible to find a working driver for your particular kernel. This problem is becoming more common as the popularity of RS-232 drops, since it means the companies providing these boxes have dwindling incentives to keep enhancing their driver to track kernel differences.
The other major type of serial to Ethernet converter is purely a network appliance. For example, with the Cyclades boxes, if it gets the IP 10.1.2.3 from the DHCP server, you can connect to 10.1.2.3 on TCP port 7001 to connect to the first serial port. You'd use TCP port 7002 for the second serial port, and so forth.
To set serial port parameters with this sort of converter, you typically have to use a web management UI hosted by the converter box. While this does mean you don't get features like automatic serial port parameter forwarding to the converter, you do get compatibility with any program that can open a TCP connection without needing a driver.