So what exactly can biotics do according to lore.

Apparently they turn blue when they are drunk and have sex, so that's a cool bonus.

Short (somewhat snarky and dismissive) answer: read the codex.

Long answer:
Biotics is the ability of some lifeforms to create mass effect fields using element zero nodules embedded in body tissues. These powers are both accessed and augmented by using bio-amps. Biotic individuals can knock enemies over from a distance, lift them into the air, generate gravitational vortices to tear obstacles or enemies apart, or create protective barriers.

There are three branches of biotics. TELEKINESIS uses mass-lowering fields to levitate or impel objects. Mass-raising KINETIC FIELDS are used to block or pin objects. DISTORTION uses rapidly shifting mass fields to shred objects.

Flare and Reave can be considered DISTORTION, which is about damage. Any secondary effects (such as Reave's damage protection or health steal) is purely gameplay and not consistent with the lore.

Barriers are likely a product of the Biotic themselves, but is likely bolstered and given form through their armor. Mass Effect field manipulation is primarily what makes a Biotic who they are, but it isnt limited to just them. It's possible to apply the science into machinery and warfare. Whether these Barriers are intentionally constructed or not (I - personally - think they are) is not explicitly stated. If it is something you need to "Turn On" then it would probably be too taxing at certain point. I believe Jack mentions that she needs several thousands of calories in order to make up for all the Biotic ability usage she does, so I have to imagine keeping your Barrier up would tire you out at some point.

Another thing to note is that Biotics who are advanced to use "Blink" (ie. Passing through objects) will be rendered sterile after doing so.

In theory, yes. But probably only for a short time. Minutes, not hours, I would think.

Using biotics take up a LOT of personal energy. As noted in the codex:

Because of the massive physical effort required to biologically generate dark energy, biotic soldiers are assigned a large daily calorie ration (4,500 kilocalories per day, compared with the standard soldier's allotment of 3,000) and provided with energy drinks to help them maintain their blood sugar and electrolyte levels.

And this is WITHOUT keeping a biotic barrier up for hours at a time.

Yeah we all know that a ttheend of the Biotic shield run in the suicid emission that the biotic is tiring at that point an dcan't hold the shield much longer.I don' tthin kshielding yourself neccessarily would take up more energy obviously than a standard kinetic barrier does tha tth eaverage soldier wears. It only becomes a strain when the yhave to create bigger ones hta tshield a group such as what the biotic shield person does inthe suicide mission.

Yep. There's even fan art out there where Shepard is a warrior while Kaidan is a mage. Parallels are pretty big. Now, if they ever did an ME/DA crossover beyond a couple of easter eggs it would be interesting.

I think it's the Liara sex scene in ME3.

I think it's also mentioned in context of Kaidan somewhere in ME3 if you romance him. I just can't remember where. 

It's not part of his romance. It's if you invite him to the party but leave Jack out. If Jack is there, she's the one that answers James about what happens when a biotic gets drunk. If she's not there Kaidan does it.

That's how I saw it too. Kaidan is my definitive love interest so I knew it wasn't connected to that.