Well; there is my sarcastic answer: pm is when I drink wine and am is when I drink coffee or “does AM actually exist”? Being that those answers are unhelpful in the extreme, let me give you the actual, helpful answer.
12 hour clock
AM stands for ante meridiam, a Latin term meaning “before midday” and PM stands for post meridiam meaning “after midday”. When we use the 12 hour clock, it is necessary to delineate the individual hours as they occur twice in one day. It is not precise to simply say “one o’clock”because your listener won’t know if you are referring to one o’clock in the morning or one o’clock in the afternoon. In addition, it is not correct to assign AM or PM to 12 o’clock midnight or 12 o’clock noon. These times are neither AM nor PM, however as little as one second after either one can accurately be referred to as AM or PM. For example, 12:00:01 pm means exactly one second past 12 o’clock noon.
24 hour clock
Years ago, as an event planner working in the travel industry, I grew accustomed to using the 24 hour clock to avoid any confusion about what part of the day I needed to greet my clients at the airport. Confusing at first, the 24 hour clock is what airlines use to list their arrival and departure schedules. A 24 clock simply means that each hour in the 24 hour day has its own number that occurs only once a day; starting at 0100 (one o’clock in the morning) and concluding at 2400 (midnight). The numbers increase sequentially from 0100. Where some people get lost, is the numbers after 1200 (noon), but it is quite easy to calculate. Simply subtract 1200 from any number after 1200:
1300 hours in the 24 hour clock is one o’clock pm in the 12 hour clock. (1300-1200=100 or 1pm).
1400 hours in the 24 hour clock is two o’clock pm in the 12 hour clock. (1400-1200=200 or 2pm) and so on until you reach 2400 (2400-1200=1200 or 12 Midnight) and then the cycle repeats itself.