Moving vertices that aren't selected. Need help.

I have been working on a model. Selecting vertices and extruding along the way. I hit something on accident and now when I move/scale selected vertices, the movement affects other unselected vertices near the movement. What did I do? How do I get back to only moving/scaling the vertices I have selected?

Please help.

You probably hit letter “O” and turned on proportional editing. Try “O” again.

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