A 16" wheel allows these bikes to fold down a little more compact the 20" wheeled bikes, although you do perhaps lose a little of the feeling of stability that the larger wheeled folders promote in the rider.
Generally quite simple, they are perfect for around town and relatively maintenance free with simple enclosed hub gears.

Probably the most popular choice amongst most riders.
The 20 " wheel gives a more 'normal' feel to the bike, while remaining small enough to allow a compact fold for off the bike convenience. Many of these bikes also feature a large range of gears making them the perfect choice for commuters in hilly areas, recreational cyclists and touring cyclist who want to integrate there riding with public transport.

Basically a full size bike that folds in half. If a compact folded size is not a priority, then these bikes are just like riding any other, except you can fold them up and fit them in the boot of your car, or stash them under the stairs if storage space is an issue.