Thin headed arrows show the individual forces that combine together to give the triangle headed resultant force. A blue arrow shows the speed and direction of an object's velocity. Unless a simulation prevents it, you can change an object's velocity while the simulation is paused by pulling the velocity arrowhead with the mouse. In some simulations you'll see a bar across an arrow. In some simulations the bonds between objects are colored to indicate if the bond is compressed or stretched.
Stretched bonds turn first dark then light blue.
The mouse can be used to interact with and edit simulations in most 2D views. While the simulation is playing, you can grab hold of an object and move it around with either the left or right mouse button. Moving an object with the right button stops it being affected by other objects and also sets its velocity to zero. Holding down the Ctrl key causes selected objects to be copied rather than moved. Multiple selections can be made by Shift clicking, or dragging out a rubberband over a group of objects.