Here is a list of useful knots with pictures which are used in sailing together with some tips about their usage on your dinghy.
Most ropes used these days in dinghy sailing are made from synthetic materials. Ropes can be made of polypropylene, polyester, nylon and new materials such as high modulus polyethylene, Liquid crystal polymer (LCP) and PBO. Most ropes are braided and some have a parallel core for additional reinforcement. Manufacturers usually provide detailed information about their products as the diameter, materials, braiding formation and other factors can have an impact on the strength of ropes.
Figure of eight Loop – This is a simple way of getting a loop in a piece of rope, although it is not as easy to undo as a bowline it is a very secure knot. 
Chain Sennet – The sennit is a series of loops joined to form a plait, this is usefull for forming handles on ends of rope. This is often used on the new kicker and outhaul systems for a Laser dinghy for example. It can be made into a loop by tying a bowline with the loose end and the part before the sennit.
Stopper Knot – This knot is used a substantial way of finishing of a rope end and stopping it from passing through a cleat for example
Sheet Bend – This is one of the most common knots for joining 2 ropes and is simple to tie. A double sheet bend is better when joining 2 ropes of unequal diameter.
Reef knot – So called because this knot was originally used to reef the sails on yachts, this is useful for joining two pieces of rope or as a binding knot.