In this module, you can study how to find the area of squares, rectangles and triangles; and the differences between perimeter and area
The video above contains a 12-minute long explanation from Khan academy covering the basics of area and perimeter. This is by far the best video I could find using the metric system, as it does a good job of explaining the meanings of both area and perimeter and the differences between the two. However, Khan does make two mistakes in the video. The first one is an addition error (!) in the video at about 4:20, when he says 8+8+8+8 is 36 instead of 32. ALSO, there's another mistake at about 9:20 where he gives an area as "centimeters" instead of "centimeters squared". Even maths teachers make mistakes. :)
The above video contains a very clear explanation about how to find the area of triangles.
This page from mathopenref.com explains how to find the area of a rectangle and then allows you to design rectangles and find their area. (NOTE: If the virtual manipulative does not work for you, try clicking the "full screen" button.)
This page from mathopenref.com shows you how to find the altitude (height) of a triangle and how to calculate the area. You can test knowledge by designing triangles and finding their area. (NOTE: If you have trouble with the manipulative, try clicking the "full screen" button.)
