A vented tumble dryer discharges the steam outside the unit, so unless you want your house full of steam you must connect it to the great outdoors somehow.
Some use an extending 'elephants trunk' that hangs out of the window, others have a permanent connection through an outside wall.
A condensor dryer turns the steam into water, which is collected in a tank within the dryer.
For years I had a vented dryer and was very pleased with it - I would never have had a condensor dryer.
Circumstances changed and I could not site my drier anywhere near an externalwall or window and was forced to buy a condensor.
I would not go back! It is superb - but I believe the running costs MAY be higher. This is not the case for me because the sensor is very accurate and stops as soon as the washing is dry.