The former is a main quest, and the latter is a secondary quest in which you help Triss out with a problem.
The first few things you need to do are to complete the quest A Favor for Radovid, and then A Matter of Life and Death. If you want to attempt to woo her then you’re going to need to treat her well, otherwise, she’ll leave you. She’s actually an integral part of the in-game history, and she is one of the main two romance options for Geralt. Triss is the red-headed sorceress who has been present in all of The Witcher games.