Saga Quests

The Saga Quests are long quest chains that are not tied to any zone. The quest chain will usually take you to many different zones and locations and will often have group content (dungeons, raids, etc) towards the end of the questlines.
Saga questlines often give beautiful unique weapons as rewards, so even if the content is no longer relevant to you, the quests are worth doing anyway to get those lovely weapon wardrobe skins!

Lost Questline

If you lose track of where you are on a saga questline there is an NPC in Tempest Bay who can help you..
Gold Tongue stands just outside the throne room at
/setwaypoint 12953 11560
When you talk to him, he will have conversation for each of the Saga Quests you are currently running, clicking these conversations will give you hints for the next questgiver in the chain.