Where is the use of pressure,level,flow switches? and what is input and output?

Pressure switch used on pressure line , Level switch used on Drum level and Flow switch used on Air flow line Input of these switch is24VDC and output is potential free contact 

These are the signals which we are going to control

pressure and flow level are the inputs In0-4ma or4-20ma.flow switch is output in24vdc

Dear when we talk about digital signal handling in automation we normaly use feils instrument call switches ...now if your measuring level of oil tank you will prefer to have level switch ,that can be high level ,low level and medium level switch,same if measing pressure of5 PSI u installed switch if your value reaches up to that5 PSI it should give signel to contoller,now u will use presure switch to acheive that goal ....its output can24 VDC

Switches are different then transmitters. Switches are only contacts (Either Dry or Wet depends on application) having NC/NO position. It activates only when process parameter reaches to pre defined values. Output can be either24V DC or230V AC depends on your requirement.

Eg. You have a pressure switch installed on pipeline with set point1 bar having NO contact, then when pressure goes more then1 bar it will be NC and will give signal to your control system.