What is the difference between a CSU/DSU and a modem?

CSU/DSU converts digital signals from a router to a phone line; a modem converts analog signals from a router to a phone line.

A CSU/DSU (Channel Service Unit/Data Service Unit) is a digital-interface device used to connect data terminal equipment, such as a router, to a digital circuit while.

A modem which is a short for "Modulator / Demodulator." It is a hardware component that allows a computer or other device, such as a router or switch, to connect to the Internet. It converts or "modulates" an analog signal from a telephone or cable wire to a digital signal that a computer can recognize.