Geometry: Triangles Perpendicular Bisectors Altitudes and Centers

Does anyone understand how to get the circumcenter, centroid, orthocenter, incenter, how to know prove if something is an altitude, a median. My teacher didn't go over it very much at all. Think someone could explain it to me?

Sketching Lines
Drawing a Perpendicular Bisector:
http://www.nexusjournal.com/GA%20Images/GA-3.3-1.3.gif
Bust out your compass and set it to a radius which is slighlty longer than half the length of the line your bisecting (don't bother measuring, you can do it by eye). Use the endpoints of two lines as centerpoints and draw cicular arcs. The arcs should be long enough so that (A) they both intersect the line and (B) they intersect one another twice.
Finally, join the intersections of the arcs with a straight line. It will pass through the middle of your original line at a 90 degree angle.

Drawing an Angle Bisector:
http://faculty.prairiestate.edu/skif...uct/con3_4.gif
Again using you protractor, set a similarily large radius as above. Using the the vertex of as your center, draw a circular arc wich passes through the arms of your angle.
Shorten up the radius of your compass. Now, using the points where your oringal triangle intersected the arms of your angle, draw two circles.
This process will result in two circles which intersect in two places, use a straight line to join these points and the vertex of your angle.

Drawing a Traingle's Median:
Find the center of a traingle's side a, either by measuring it out with a ruler, or using the compass stratagy outlined above. Join this midpoint with vertex A (the vertex directly opposite side a) to create a Median.
After completing this process, you will beable to identify other meidans.

Finding Points
Drawing a triangle's Circumcenter:
Begins with drawing the Perpendicualr Bisector of the triangles three sides. When you have done so, extend these new lines towards one another and they will soon interesect. This point of triple intersection is your circumcenter.

Drawing the Triangle's Incenter:
Work's in the same way, simply use the traingle's angle bisectors, rather than the PBs of its lines.

Drawing the traingle's Centroid:
Work's in the same way, simply use the triangle's medians, rather than the PBs of its lines.

Recognising Altitudes and Orthocenters
Altitudes:
Are lines which are perpendicular to a triangle side (usually noted by the right angle symbol) and pass through the opposite vertex.
Orthocenters:
Are the intersections of an triangle's three altitudes.

There are also 3 excenters which you get by finding the external angle bisectors...

Just extend the sides of the triangle and bisect the two pairs of angles that are formed on each vertex, 1 is internal, the other external. The external angle bisectors are perpendicular to the internal ones. The intersection of one internal (say, the angle at vertex A) and the two respective external bisectors of the other two angles (at B and C) is called excenter.