To put it simply, a variance is how you ask the city to break the rules! When you are developing your design and working with current zoning bylaws, but you find yourself needing a reduced set back line this is where you would want to apply for a variance. This is done with your development permit and is required to go in-front of council. You would have many discussions with your city representative that is handling your file and they will be able to advise if it is supported or not.
If the city supports your request than this is where strong public relations is important. Once it is in front of council if there are community members that do not support your request they will present their case at council. Getting the support and buy in of your neighbours and any properties effected by your development is key to getting any development permit excepted especially one with Variances.