Their are many different types of Trance, multiple subgenres. The sounds vary from area to area.
Cascada is more of a pop song, trance song and happy hardcore having some fucked up child together.
Here is a good guide to electronic music, it will help teach ya more, + their's alot of good artists on here. You will also notice with the electronic and dance music scene you will be able to find many versions and remixes of songs. Vocal mix, instrumental mix, Club Mix, Radio Edit. And many remixes by many artists. Generally the general public wants the radio edit or origional mix
Okay for some songs. These are what I have just bought, and are really good songs.
Beats at Rank-1 Dotcom - Rank 1
Ferry Corsten - Fire
4 Strings - Turn it around
Perasma - Swing 2 Harmony
Marcel Woods - Advanced
Dumonde - God Music
ATB - Ecstasy
Stunt - Raindrops
And the two most popular all times classics are "Darude - Sandstorm" and "Zombie Nation" (I think the actual song is "Zombie Nation - kencraft 400", however it is just refered to as "Zombie Nation"). However most trance fans would shoot you for playing either one of the songs.
Than the most popular DJ / Producer is DJ Tiesto. However everyone either loves him or hates him. Im in the middle, I like his stuff but he isnt the god that most people think he is.
And here's some other stuff I like, bit less recent.
Deep Dish - Say Hello
Lost Tribe - Gamemaster (One of the best damn songs ever IMO)
Darude - Out of Control (Ducks because of bullets)
Bruno Fergani - Trance of Love
Dirt Devils - Music is life
DJ Aligator - Secrets of the space
Well their's some of my stuff, I have bought copies of almost all the songs listed, however the older songs are almost impossible to come by on digital formats.