This is a MUST in a visit to Lisbon.
For 18Euros, it sounds a little expensive but for what you get this is one of the best deals in town. As for 24Hours it allows you to go on all the other trams and lifts. So you can start with this one, then go until the Elevador da Bica and go uptown again. And just walk down to the Elevador de Santa Justa. These two are my favourites, but there are a couple more and you can use your ticket in all of them.
The tram departs from Terreiro do Paço every 20 minutes. I recomend you get the normal seats on the letf side as it will allow you to see everything without a problem.
You will pass throught the most important parts of Lisbon. You can exit the tram on several stops along the way and resume it when the next arrives if you want. But my personal recomendation is don't. Do the trip all at once, and signal on the map that they provide the places you want to go and visit again. Then when you arrive at Terreiro do Paço again ask the clercs what other Tram will take you to the places you want to see again and just hop on it, IT IS ALREADY PAYED FOR ANYWAY.
It can be a little bit boring for little children. But is a lovely ride for adults.
I hope this helps you.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.