As a result of it being old, the metro stations are sometimes small, stifling and dirty, and quite frequently you'll see rats on the tracks.
The New York City Subway has 26 lines referred to by numbers or by letters. You can check out the metro map here:
Both the subway and the bus system operate on a 24-hour basis. Nevertheless, not all the Subway station entrances are open all night nor do all lines run throughout the night.
Normally, the subway lines run every 2 to 5 minutes during peak hours (from 6:30 am to 9:30 am and from 3:30 pm to 8 pm). During midday, they run every 5 to 10 minutes and between 5 and 15 minutes till midnight.
From midnight until 6:30 am, trains run approximately every 20 minutes.
Certain subway entrances are a little peculiar and one has to be careful when choosing the entrance of certain stations, as some will take you directly down one direction (ie north or south), so you'll have to get out and find the correct entrance if you realise you're going the wrong way; so make sure you take the correct entrance.
It is also important to keep in mind that there are two different types of trains: normal and express. The express trains don’t stop at all stations en route and are a lot faster, so therefore are very useful when traveling long distances. However, if you decide to get on an express train, make sure it stops at your stop before getting on.