Drum vacuum cleaners are heavy-duty or industrial versions of cylinder vacuum cleaners.
Cylinder consists of a large vertical drum, which can be on wheels or even stationary. Power comes from larger electric motors or they are connected to central air compressor and they use Venturi effect to produce suction power.
Drum vacuum cleaners are mostly used in hotels, larger homes and similar areas where there is need for regular cleaning of larger surfaces. Models that can store up to or even more than 200 liters (~ 55 gallon drum vacuum cleaners) are not uncommon.
In order to cover as large area as possible using single outlet, drum vacuum cleaners usually have very long power cords with automatic rewind - power cords of 10-20m (30-60 feet) are not uncommon. Due to the strength of motors (and hence increased currents) and due to the length of power cords, power cords are thicker, more robust and require more space for on-board storage.
Length of hoses depends on the models, but 3-5m (10-15 feet) are normal for drum vacuum cleaners. Since they are used on various surfaces, they are supplied with numerous attachments that are usually carried on-board for faster switching. Having specific attachment for different surfaces requires time for swapping the attachments, but improves cleaning and in the long time, saves time!
Drum vacuums incorporate various filtering systems and filters that can filter large volumes of air before they require replacement or cleaning. High-end models have HEPA grade filtering and are suitable even for hospitals - hospital grade drum vacuum cleaners (with lower levels of noise, too) can be pricy, but they get their job done.
Wet/dry and shampoo drum vacuum cleaners use various liquids to improve cleaning - liquids are usually water with some cleaning agent added to the water. Also, such vacuum often use water for additional air filtering - emphasis is here on 'additional air filtering', since water is a good filter for various dirt and debris, but for smaller particles water is simply not good enough alone. But, water filters combined with mechanical air filters or even cyclonic filtration provide very high level of air filtration.
Add a good UV lamp into the combination and exhaust air flow will be practically sterile and without particles that can cause any health related issues.
Such combinations are possible on drum vacuum cleaners due to their initial size. Of course, all of this increase their size, weight and price.