<p>My first and only 3D printer is a Printrbot Simple Metal, which has a hotend that doesn't expose any of its internal parts. Easy for beginners, I suppose: "The hotend is that tube that heats up the plastic and deposits it on the print bed."</p>

<p><a href="https://i.stack.imgur.com/7y1yy.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/7y1yy.jpg" alt="Printrbot Simple Metal hotend"></a> <a href="https://i.stack.imgur.com/SwwW1.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/SwwW1.jpg" alt="More complex looking hotend"></a></p>

<p>But I've been trying to learn more, and many hotends out there don't look quite as simple. My Printrbot hotend probably isn't as simple as it looks, either.</p>

<p>What are the parts that make up a hotend, and what do they do?</p>

<p><em>(PS: This is a general question, not specifically about the two example hotends above.)</em></p>

picture on the left
This hotend is made out of
PEEK plastics
(beige). It can work in temperature upto 250C. As it is also good heat insulator then it doesn't need to have cooling fan. Red part on this picture is just a cover (insulator) of the heater which heatup nozzle (gold). Black are just wires and connectors.
picture on the right
This hotend is metal so it needs two things - cooling fan and heat barrier. Beginning fron the left - red wires are connected to (silver) block which is heater itself. Between the block and this round silver part (which is a heatsink) there is heat barrier (thin pipe which is poor heat conductor). Heatsink has plastic funnel (blue) which directs an airflow from fan (black) through heatsink.