What's the Output Voltage on Pins?

Hello! I'm new here and new to Arduino.

I am trying to run a small motor using an Arduino unit. The motor runs great when hooked directly into the 5V power pin; however, when plugging into a digital pin set as OUTPUT with digitalWrite HIGH, the motor spins much slower. Half speed, if I was to estimate.

Here's the twist: a multimeter shows the voltage of the digital pin to be just under 5 volts. What am I missing about how these output pins operate?

What am I missing about how these output pins operate?

You're missing the amount of current they can source or sink, and the potential damage you've done to your output pin.

How is this an installation question?

AWOL:
You're missing the amount of current they can source or sink, and the potential damage you've done to your output pin.

How is this an installation question?

Sorry, I saw "troubleshooting" and thought that might include information on having trouble with the hardware itself. If this is the wrong forum, I hope it can be moved somewhere more appropriate.

When you say damage to my output pin, what do you mean? And how much current can they source or sink (and why, then, would the multimeter still show 5v)?

EDIT: Ah, I see here that something could draw excess current and damage the pin. I'll have to check and see what the current draw on this tiny 10mm DC motor is. Thanks for your help!