What is the difference between an amplifier and a speaker?

An amplifier takes a signal and makes it louder. Usually you connect an amplifier to a speaker to give the speaker a signal that can actually be heard. If you were to connect your iPhone to a speaker without an amp, you wouldn't hear anything. Many speakers have amplifiers built in which is why there may be confustion. A speaker is the piece of equipment that takes the electrical signal from an amplifier, and makes it into sound.

The amplifier is what drives the speaker. It sends amplified voltage to the voice coil of the speaker, which is attached to the speaker cone. This causes the speaker to move in relation to the standing field of the magnet. Google "how speakers work" if interested.The way a stereo works is, it takes a small electrical signal generated by, say, a phono cartridge, amplifies that to what is called "line level", then passes that signal to the amplifier which, yes, amplifies it, and in turn drives the speaker with that electrical signal. Of course, it's a bit more complicated than that, but them's the basics. Sometimes these functions are combined into one component, (receiver, integrated amplifier), or done by separate components, (preamp and amplifier). A simple device like an iPod performs all of these functions, and in turn drives attached speakers, (earbuds or headphones), just on a smaller scale.

amplifiers increase voltage gain or current gain or both. Speakers are electrical to sound transducers... they make noise when driven by an amplifier that is fed with sound signals.

An important difference is the amp's job is 2- to 2-dimensional (amplitude changes over time) while a speaker take a 2 dimension signal and convert it to the 3 dimensional sound wave leaving the speaker that "fills" the listening room. I left out time as a dimension in the room, you may add it if you want. The picture of the 3-D space of a room compared to a curve plotted on a flat surface's (a display or paper for example) 2 dimensions illustrates my point. The spekars job is more complicated and needs to be designed for this. Turn your spekers backwards first and then try the same with your amp (back side pointing forward) and you'll see (understand) how the amp is unaware of the space the speakers have to deal with. The amp also see's the load the speaker put on the amp's output, or whatever an amp is connected to, but these are made to be of no problem for the amp if wisely done and can therefore be ignored here. We have only talked of a 1 chanel single amp and single speaker up to this point, but usdually we have 2 or more channels. Left/right 2-chanel stereo is the most common today and the two speakers act together in the room and need to be designed for this while the 2-chanel or stereo amp ignores this. There is devices that like the amp is 2- to 2-dimensionally working that take room parameters in consideration like reverbs for example. But not the amp!

Amplifier is a DC to AC power converter with a smaller voltage signal controlling the conversion. Speaker is a transducer, converting electrical vibrations to mechanical vibrations.