Phasing has a significant impact on the grouping aspect of PVE gameplay so everyone needs to know how it works.
Zenimax's challenge when creating ESO was figuring how to combine the single player aspect of the TES series with the traditional aspect of MMOs (ex. group dungeons, PVP, raiding). End result is that pretty much all of the PVE content that you do to level up is designed for a solo player to complete. Just like the TES single player games, you are the hero of the ESO story and all of the quests are written to advance your individual story. There are exceptions in each zones (skull bosses, dark anchors, group public dungeons) but for the most part everything else in a zone is designed for a solo player.
A significant amount of this solo PVE content is phased. By phased I mean after you complete the quest, you are put into a different phase of the zone. For example, lets say you go to the village of OTGville and get a quest to determine why the residents of OTGville have been acting so strange lately. If you open up the map and hover your mouse over the map icon for the village of OTGville, you will see a message "Determine why the residents of OTGville have been acting so strange lately". During the course of the quest, you discover the leaders of OTGville are vampires and have the entire village of OTGville enthralled. While you are doing this quest, you end up fighting vampires and their minions. After you destroy all the vampires and save the residents of OTGville, you are put into a new phase. In this new phase, the vampires and their minions are no longer present. Whereas before everywhere you went you were fighting vampires and their minions, now there are no mobs present that attack you. When you go to the map and hover your mouse over the OTGville map icon, the message now reads "You saved the residents of OTGville from the vampires".
What is the impact of this on PVE gameplay? For the solo player, it helps increase the immersion factor because you get the feeling that your action are having an impact on the gameworld. Almost everyone I have played with in the closed beta liked this aspect of phasing. Phasing however can create some significant problems when it comes to grouping. Lets say I have already completed the quest and have saved the citizens of OTGville. Another guildmate starts the quest but is having problem defeating the final boss vampire so they ask for help in guildchat. Because I have already completed the quest and am in a different phase, I can not come to help them because they are in an earlier phase of the quest. If we join group and I come to their location, we will be invisible to each other with the only thing displayed is the group member icon over our heads. The ONLY people who will be able to help the person complete the quest are players who have not completed the quest themselves.
Another impact of phasing I have seen is in the group dungeons. For example, in Crow's Wood at the end you are presented with a choice of who you will side with, NPC A or NPC B. If you are in a group, the selection each player makes determines what phase you will enter. If I side with NPC A I have to defeat NPC B while if my groupmates side with NPC B they will have to defeat NPC A. When these choices present themselves, you need to make sure everyone is on the same sheet of music because if you side with NPC A and everyone else sides with NPC B, you will find yourself fighting the boss all by yourself and possibly unable to defeat them.
I saw in zone chat how to change phases. Open the "o" menu, right click the name of the character you want to join in phase and then "Travel to Player" ... Which would place you at a wayshrine closest to the player in question.