All airlines run a staff travel programme of one sort or another. Airlines will give a discount generally calculated as a % of the IATA price for tickets on the airline they work for, or as per mile charge for the ZED fare scheme (zonal employee discount). Tickets are bought in advance and have all taxes and surcharges added after % is calculated.
All these tickets are standby, in otherwords while it appears to be a fantastic perk you can easily end up left on the tarmac. Generally to guarantee a flight you have to buy tickets on several routes then turn up to the airport and hope, for those with families this can mean it is rarely worth using since the chances of 4 or 5 people getting on can be very slim.
Often retired parents are the people who can make best use of staff travel since they are rarely time constrained.
Cabin depends on company policy either seniority or length of service, ZED fares on other airlines are almost always in economy.
Employees can also buy firm tickets at 50% of IATA though to be honest that is often the same price as a market fare.
Selling tickets is an instant dismissal, doesn't mean it has never happened but generally it is not ever worth it. Single staff can fly their brothers sisters and parents, married their own family and parents (these policies vary on every airline.)
Oh and they take this benefit into account when setting staff wages one of the reasons why airlines rarely pay as much as people think.