Bid / Ask Price

[hagadol]

What controls the Bid and Ask (Purchase and Selling) price difference and how do the move (in relation to each other) as the price of the stock moves ?

 

[TWI]

Buyers and sellers control bid/ask
More buyers bid moves up ask withdraws
More sellers ask moves down bid withdraws
Always be a spread as many rather pay a tick below offer or sell a tick above bid. Market makers profit from this.
In illiquid market bid/ask gets wide as at times nobody is keen to trade.