A 24 hour watch is exactly like any common (12 hour) watch with the only difference that in this kind of watch there is one hand that completes a full rotation in 24 hours instead of 12 hours. Most of the 24 hour watches have two hands to indicate the hours: one is based on the standard 12 hours cycle while the second one completes the rotation in 24 hours. Some, instead, have only one hand to indicate the hours and this is the 24 hour hand.
The 24 hour watches are sometimes used to show two different time zones (e.g. GMT watches), to avoid confusion in the reading (e.g. military watches), to sync with a particular lifestyle or just to meet specific people preferences.
The hand that indicates the hours takes one entire day to complete a full cycle in the dial and depending on the brand people might find 2x12 hours or 1x24 hours indication marks. Also, some brands indicate midday at the top and midnight at the bottom while others show exactly the opposite. The first Todd & Marlon Collection (#YOURTIME) uses 2x12 hour marks with midday indicated at the top via an applied silver sun icon and midnight at the bottom as indicated by an applied moon silver icon. Hence, on the left hand side there are the morning hours (1-12) and on the right hand side the afternoon hours (1-12) - both clearly indicated by the main numbering system.
In the 24 hours watches the minute and seconds read in the same way as in the classical watches – though some 24 hour watches have no indication for minutes and seconds (e.g. slow-watches, Botta Design). In #YOURTIME watches both minutes and seconds are indicated with hands pointing to a secondary numbering system (red numbers). Interesting to note that the main numbering system which relates to the 24 hours can also be used to read minutes and seconds if one consider that each mark relates to 2.5 mins/secs.
In this video you might appreciate the difference between a 12 hour and a 24 hour watch... and if you want to know more about 24 hour watches you might have a look at the website 24hourtime