Unlike road cleats, MTB (2 bolt cleats) are smaller to allow for the space they slot into on the sole of MTB specific shoes. Using this style gives the rider a secure connection to their pedals resulting in efficient effort transmitted through the cranks. Whilst going over rough terrain your feet will stay attached to the pedals which makes for a better ride maintaining control. However, you can easily unclip when needed.
With the cleat being recessed into the sole of the shoe, it enables ease of walking when you need to dismount from the bike.
What is Float? Float is the degree of movement given by the cleat, or how securely it is positioned. This allows your feet to either pivot or not when pedalling. Some cleats have no float which keep feet locked into the pedal. This may be preferable for racing, but a system with float maybe a better option. This allows joints to move freely when pedalling and help prevent strains.
When thinking about your choice of pedals you might want to consider which shoes typically accompany them. You may already have a pair of shoes with cleats and consideration must be given to the compatibility, as there is little crossover between pedals, cleats and shoes. Therefore it is best not to set your heart on a particular pair of shoes or pedals you love before finding out if they are compatible.
If opting for cleat based pedals, you may be one of the lucky ones who can attach and attack, or you may need to play around with adjusting them every few rides to get the right position for your pedalling. It is worthwhile having a cleat fit but this isn't always necessary.