There are essentially two parts to the "Main" quest in ESO:

1. The part that is specifically called the "Main Story", which you get every 5 levels (at Level 5/10/15 etc) from the Prophet in the Harborage;
2. The part that is the main storyline for your alliance, called the "Alliance Story", which takes place in the Alliance Zones. These are chained together one after the other.

It is hard to tell in-game whether a quest is part of the zone's Alliance Story or if it is a Side Quest in that zone, as they are both just listed under the zone name.

When you reach Level 43, the Main Story and Alliance Story join together to lead you through the final levelling zone.

For more info, see the UESP pages on Main Quest and Story Quests
For details on all quests, Start Here.

nunoheadbanger wrote: »

how do I know what the main quest? symbols?name?

In the Journal, the "Main Story" part of the Main Quest is always listed in the "Main Story" section, and on consoles also has a specific icon (see left).

The "Alliance Story" part of the Main Quest is not directly identifiable, as it is marked in the same way as side quests in the same zone; listed in the Journal under the Zone name and with the Normal Quest icon. Because of this, it is not possible to tell the difference between Alliance Story Quests and Side Quests. However, if you do the Alliance Story quests in order, completion of one will almost always lead to the next one, and the first quest you see when leaving Coldharbour is the first quest in the Alliance Story.

See the links above to UESP for details on the quests.