from http://heroesofthestorm.gamepedia.com/Tassadar
Tassadar, the Savior of the Templar, is a support hero from the StarCraft franchise. An executor without equal, Tassadar fought valiantly to save Aiur from the Zerg. Channeling the Khala and Void, he destroyed the Overmind at the cost of his life. In the glory of combat Protoss warriors forever invoke his name: "En Taro, Tassadar!"

Tassadar can grant damage-absorbing shields to his allies, detect nearby enemy heroes, and manipulate the battlefield for his team.

You can learn more of his lore there : http://starcraft.wikia.com/wiki/Tassadar

Tassadar is a range support in HoTS. He has access to some support talents (obviously), like Healing Ward and Calldown: MULE. He's considered (like Tyrande) as a Support that have a good DPS output.

Plasma Shield (Q) is a REALLY STRONG shield. He will save your life often, and allow you to almost never get damage when laning. Don't forget that you can ALT+Q to autocast it on you.
Psionic Storm (W) is the classic High Templar attack. It deals damage when cast, and damage for those who stand in it. It is amplified by Static Charge (lvl 7) that make your next autoattack to crit the enemy hero/structure/minion. If the enemy stays in the storm, he'll take crits for every autoattack you make.
Dimensional Shift (E) is your escape. Not upgraded, he will allow you to avoid some damage, and perhaps save yourself. If you pick Prescience (lvl 13), it gets an autoactivation when you are under 15% life with a separate cooldown, allowing you to have multiple shifts in the same fight (and with Dimensional Warp, it's synergies a lot !). You can avoid some damage by manually activating this talent, like Zeratul's Singularity Spike per example : if the damage procs when you are shifting, it nullifies.
Archon / Twilight Archon is the 1st heroic ability. You get a huge armor, huge skin, huge damage + AoE ! Really strong heroic ability and an important part of Tassadar dps output.
Force Wall / Force Barrier is the 2nd heroic ability. Double-edge blade for me.. I don't master it AT ALL, but I tried a lot. can be really nice but I really prefer Archon in every situation.

Oracle is really nice. Allows you to know if the enemies are close to you, and where. Use it when you want to protect objectives, camps. Use it to reveal that Zeratul or Nova. Use it to scout when you arrive in a lane and you want to know if there's something in the bush !

Level 1

Good. This talent allow to have a really high mana regeneration late game.
Good. Almost everyone classic choice. Higher cast range, and that allow better harassment, and sometimes give kills too.
Situational. It depends. I like this talent on Haunted Mines when I'm defending the lane. I need this mana reduction of Psionic Storm and I can't rely on Conjurer's Pursuit.
Bad IMHO. I tried it, but not a fan. The others talents will be a better use on all situations in my opinion.

Level 4

Perhaps one day. I don't use this skill atm. I tried a "vision" build but it makes me pass some skills I want. I don't play with this, but perhaps one day I'll reconsider.
Situational. I rarely take this talent, but I remember myself picking it when playing with a melee assassin friend like Ilidan or Kerrigan. Those are situations where I prefer THIS hero to survive.
Situational. I always take this talent on Haunted Mines. Combined with mule you can nullify a (not-too-heavy) golem damage.
Bad IMHO. I don't find this talent so important on Tassadar.
Good. This talent is the best for me. You have access to it, use it. It's good for the team, even if you're going DPS, it's sooooo nice ! 30% hp for all heroes if the area.

Level 7

Perhaps one day. I don't consider picking this talent. I agree, you can pre-shield your team before a fight, that can help. But it's not in my choices atm.
Good. This is the key talent of this hero. This allows his to have a really good impact on the damage output.
Situational. Passing on Static Charge is hard, but I sometimes take this talent on Blackheart's Bay, to be an unkillable coin carrier.
Situational. I personally take the MULE on Haunted Mines, to protect the structures (check build #3)

Level 10

Good. Survivability, DPS output, Archon... Must have.
Perhaps one day. I don't like this heroic atm. Archon is really superior in my opinion.

Level 13

Good. Having a slow on your autoattack is really nice ! Good synergy with Static Charge, and allow the team to catch up some fleeing enemies.
Good. Having this Dimensional Shift autocast when you're under 15% is really nice. Nice synergy with Dimensional Warp. Remember that cooldown is not the same ! Use your (E) before the autocast. Careful.. both can activate at the same time... and you can have bad surprises.
Maybe one day. Maybe, when I'll reconsider vision builds. Lvl 13 if full of impressive talents... So I don't consider this one atm.
Bad IMHO. I know Tassadar is a little "squishy", but no, I won't take this talent. If I wan't survivability, I'd go Prescience at the same level.
Good. Classic support skill. Allow you to highly lower the dps output of a hero enemy for few seconds and slow him (to death sometimes!). Really nice !

Level 16

Situational. Used it when I was playing full support focused on helping my Illidan friend, or melee assassin like that.
Maybe one day. I don't pick this talent because I consider the others better. Having a slow AoE can help the team.. but I don't know, I don't like this one.
Good. Really really good. Check on the UI, a bar will show the time left for the free Psionic Storm. Good synergy with Static Charge. In team fights, I prefer to spread them (enemies won't stay in or regret). Versus camps or minions, wait the last second to launch one at the same spot to help yourself.
Good. Really important when you build for survivability. Good synergy with Prescience, allowing you to manually use E, shift and heal, autocast Shift and heal thanks to talent, and reuse E to shift and heal. Really nice.

Level 20

Good. The Twilight Messiah is there.. High damage, High range, High AoE, High shield... Damn.. Hard to pass on this ult.
Maybe one day. As I told for his lvl 10 equivalent.. Not for me. Sorry.
Situational. This one is good when playing in full support mode. Classic lvl 20 talents for support, and it's a good one.
Situational. I don't take this one often. Twilight Archon is hard to pass on.. But having your cooldown ready means another Plasma Shield, and another Dimensional Shift ready... That can be nice on survivability.

#1 DPS

Classic damage build. Offer a really nice DPS output. Can be used anywhere.

#2 Immortal

This build is fun, try it on Bleackheart's Bay and you'll be the best coin carrier ever.

#3 Building protection

This build is used when protecting the lane if Haunted Mines. Not a lot of damage output, but a good laner, and nice protection on buildings.

#4 DPS/Survivability

Hybrid build, allowing Static Charge with the Prescience/ Dimensional Warp survivability combo. SOLID.

#5 Full support

Rare build, but I tried this. Tassadar can be good in support, but he has to invest a lot. Not the best build IMHO, but still !
Consider not taking the Healing Ward for Leeching Plasma when you composition rely on a Illidan per example.