The diameter can be thought of as the width of the circle, more or less. More accurately, it's the distance from one point on a circle, through the center, to a point on the opposite side.
If we had a circle at point A, and some point on the edge of the circle point B, then line AB will form a third point C. Point C is on line AB and on the circle. Segment BC is a diameter.
We can measure the diameter with a straight line ruler, but we cannot do the same with the perimeter of the circle. The circumference is just a fancy name for "perimeter". Specifically, it only applies to circles. The way I remember it is that "circle" has "circ" in it, and so does "circumference". Furthermore, we can erase some letters of "circumference" to end up with "fence", which would go along an outer perimeter in a lot of cases. Overall, "circumference" makes me think of "circle fence".
How can we find the perimeter of a circle? Well we could use a cloth ruler to wrap around the edge of the circle. We'd have to make sure that none of the bendable ruler is out of place. So things could get tricky.
To find the circumference with a formula, we'll use this one here
C = pi*d
where pi = 3.14 approximately and d is the diameter
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For example, let's say the diameter is d = 10 feet. That would lead to a circle perimeter of....
C = pi*d
C = 3.14*10
C = 31.4
The circumference is approximate because pi = 3.14 is approximate. The units for the circumference in this case are in feet, so the distance around this particular circle is about 31.4 feet.