Root trainers is a proprietry brand module system that encourages good downward root growth. The sides are grooved and there is a hole at the bottom so you can see how far downward growth has progressed. Also, when the roots reach the bottom and go through the hole, they are 'air pruned', ie they stop growing in the drier environment, and this encourages secondary roots, making the plant stronger for planting out. The cells open like a book and you ease out the whole package into the planting hole. Look at the lovely root system on my first batch of peas!
Root trainers are quite expensive, but you can easily re-use them lots of times if you are careful. In addition, they are made of recycled plastic. Many people say cardboard toilet roll is a good alternative. However, you do not know what chemicals were in the pulp or what glue has been used. In addition, you don't get the air pruning effect as the rolls and compost have to sit with their bottoms in a damp tray. Lastly, the cardboard is prone to going mouldy.
The right kind of quality sticks, compost and tools really are more exciting than new shoes.