Waypoints are the graphical symbols that show up on the timetrack dialog. Each time the value of a parameter is edited in Animate Editing Mode, a waypoint will be created.
The colour and shape of the waypoint indicates its interpolation type:
Each waypoint is split into two halves. The left half indicates its 'in interpolation' and the right half indicates its 'out interpolation', so many different combinations are possible. The following waypoints are from 5 verticies of a single bline:
Note: the .sif file used to generate these screenshots is available. 
The 'undefined' (grey) symbol is used when the row in the timetrack dialog represents multiple waypoints. For example, the 'verticies' row represents all the verticies making up a bline. Each of those verticies can have multiple waypoints, each with different interpolations. If all the interpolations are the same, that interpolation's symbol will be used. Otherwise, the grey 'undefined' symbol is used.
The following waypoints are from the 'verticies' row of the same bline as before. Notice that since the left side of each of the waypoints is coloured, indicating that the 'in' interpolation of the verticies are the same for all verticies, whereas the right side is grey, indicating that there is more than one 'out' interpolation in each column.
TODO: rewrite the above so it doesn't hurt the brain so much.