HEX / ALLEN KEYS
The terms hex key, hex wrench, and Allen wrench refer to the same tool: These keys are an L-shaped hexagonal tool designed to fit and drive socket head, flat head, and button head cap screws.
These type of fasteners have a hexagonal socket in their heads. A good set of hex keys are not expensive. Good quality hex keys are usually black or in grey color because the are case hardened steel. The cheap ones are typically chrome plated and made of butter. There are exceptions of course. If you purchased a tool kit, it may come with hex bits instead of a key set. They will serve the same purpose but as with screwdrivers, I prefer to have they keys.
While using hey keys, always make sure that the key is fully seated into the hex hole inside the fastener to make sure you don't risl stripping the fastener head. If there is crud or junk inside the fastener head, clean it out first. It's quicker to clean it out than to deal with a stripped head.
Some hex keys will come with one ball end. The ball end hex feature allows you to access the fastener at a slight angle. This is a nice feature to have, just be carful when using the ball end as it could strip the fastener head more easily.
Aside from the standard keys, it is also nice to have a set of hex key sockets bits. This will allow you to use your ratchet for more leverage. If you want to spend even more money, a set of long reach hex bit sockets is a nice set of tools to have.