A clock is a device, either mechanical or digital, that measures time in hours, minutes, and sometimes seconds. 

The mechanical clock shown has a face with number 1 to 12 written around the edge and 3 hands to indicate the time of day.

The shortest hand (red) points to the hour of the day (for example, 1 o'clock, 2 o'clock, ..., 12 o'clock). When the hour hand makes a full circle around the clock once - 12 hours have passed, and since there are 24 hours a day, the hour hand goes around the clock 2 times a day. 

The longer (thick black) hand on the clock is the minute hand, and it indicate the minute of the hour. The minutes are counted using the smaller tickmark at the ouer edge of the clock. Multiply the hour numbers by 5 to find the minutes at that location on the clock  (1 means 5, 2 means 10, 3 means 15, ..., 12 means 60).  

For example: The red hand is between 1 and 2 (but not yet reaches 2), so it's 1 o'clock. The black hand is pointing at number 5, so it's 5, 10, 15, 20, 25 minutes. It's 25 minutes past 1 o'clock or 1:25.

The second hand (not shown) is the longest, thinnest, and fastest hand of the clock, and it indicates seconds. Reading the second hand is the same as reading the minute hand.