What is a ridge vent?

An old home built in late teens early twenties, unfinished attic with knob and tube wiring (in addition to newer upgrades on lower levels) has one large built in steel fan in window and one end gable, need a new roof and they want to sell me a ridge vent to go across the top. I've had recent leak with water from damaged roof, so I fear the ridge vent could create more problems with wind or rain and snow blowing in. Is my large fan (the guy that did the upstairs central

AC put it in, it's designed to automatically start at 90degrees) not enough? I don't want to overventilate, the original insulation is in the floor recessed under

the knob and tube wiring, had some blown in insulation put in 10 years ago as

well, just don't want to take any chances. Any knowledge on the safety of the ridge vent in the older homes with regards to leaks/wind/rain/snow?