Bstanko6:

In other words what does the resistor limit? Volts?

Voltage is Pressure.

Current is Flow.

Resistor is Resistance.

These are all very different aspects of electricity whose behaviors are defined by Ohm's Law.

When current flows through a resistor, the resistor provides resistance against the current flow. Ohm's law says that when you multiple resistance times current, you calculate the voltage drop. So that means if 20mA is flowing through a 250ohm resistor, it will drop 5V across it.

Voltage is measured as a difference between two points. So if you measure the voltage across a resistor, you see its voltage drop.

In a circuit voltage drops across components add up. All of the current that leaves the source has to to also enter the source.

Bstanko6:

So if I want a component to stay under 5 volts, when working with 0.20 milliamps

This very much depends on the component. Diodes or LEDs (light emitting diodes) are not linear devices. The need for a resistor is because the LED drops a certain amount of voltage and then, effectively, turns into a short circuit. The resistor is there to limit the current when that happens. In that case, you subtract the voltage the LED will drop (which is defined by its color) from the source voltage. The remaining amount is used to calculate the current limiting resistor.

For example if it is a red LED, the forward voltage is around 1.6V. So you take 1.6V from 5V to get a remaining voltage drop of 3.4V. If you want 20mA to flow through that LED, you calculate 3.4V / 20mA = 170Ohm.

Note that many people pick the maximum rated current for a LED. There's nothing that says you have to use the maximum current. In fact, allowing the maximum (or more) current through the LED will shorten its life.

On the other hand if you have a 3.3V device (like a sensor or IC), you can't simply use a resistor to drop 5V down to 3.3V, unless that device draws a constant amount of current. Any change in current will change the voltage drop across the resistor. In that case you need a voltage regulator.

Bstanko6:

Let me restate it this way, if my uno works at 5v, and my component is say 7v, how to compute a resistor to use?

That question doesn't make sense. Resistors drop voltage, not amplify it.