Kayle is usually used as an attack support character, which she does very well. But, I don't like using her for that. I'm more of a fan of using her as an aggressive healer support, which she does just as well.

I like Kayle to do a lot of healing, the problem is that until she gets her heal up to it's full level it can take a while for her heal to come off of CD. For this reason, I give her a lot of CD reduction with runes (12.21%) which will go a long way for the first few levels and make you OP in late game as you can spam cast big heals on your team mates. Along with her CD reduction I go for a little bit of mana (190 at level 18).

Kayle is full of both attack and utility capabilities. She speeds up team mates, slows down enemies and can make herself or other invulnerable for a times. I have gone with the standard mastery build with some modifications. The two most notable differences are that I've spec'd into Spacial Accuracy for the Teleport CD reduction, and Insight for the bigger mana returns for team mates on Clarity.

*Note: This is a 5's build. I wouldn't spec into either of these summoner abilities for 3's as the map is too small for Teleport to be worth it and not enough mana would be returned to justify Clarity. I use them in 5's because the group I play with uses all mana based champions and I build Kayle to have a lot of health and mana. The return on Clarity is huge for my team.

*Note: It's necessary for Kayle to be able to jump from one side of the map to the other in early and mid game. She saves team mates from impending doom or can even help with a gank. Teleport is a must have if you plan on playing healer.

I build Kayle to be able to weather the storms of combat with lots of health and mana. Too often do I see support characters who have to hang back because they get ripped apart when they get close to combat, especially during the laning phase. Needless to say, that doesn't work so well when you need to get levels. Kayle should be built to stand up to ganks while pulling herself and her laning partner out of the fire. That being said, allow me to justify my items.

*Edit: I changed the line up to include Tear of the Goddess right after the boots. The benefit from getting the 1k extra mana stack rolling early is too good to pass up.

The 3 items after the initial purchase of the Doran's Ring are the most important ones. Again, I'm obsessed with CD reduction for healing, so I get the Ionian boots because this gives me 40.21% CD reduction overall and it's the first real item I've purchased. This is a huge advantage at the beginning. You'll go oom fast, but that's why you have clarity. The Rod of Ages is an important piece for you because it gives you that health/mana/AP boost early on. Rylai's Crystal Scepter is the thing that seals the deal for your survivability and utility. Health, ability power and the slow that goes along with the Scepter give you your aggressive edge. You already had a slow with your Q, but now it's an even bigger slow. You've got just over 2-3k health (depending on level) and mana to spare, and you can spam moves like none other with your CD reduction. You're set to go off to team fights.

The last 3 items are situational/preferential items. You could argue for picking up Archangel's Staff earlier in the game for the stacking mana buff, but generally it's not needed until later in the game when you're getting involved in team fights (which should be around the time you pick it up). Morello's Evil Tome gives you that sweet AP for those bursty heals and killer damage on your Q, and the mana regen doesn't hurt either. When I get to my last item, there really isn't anything else to build. You could go with the D-Cap for more killer heals and dps, but I went with the Soul Shroud, just to be a jerk. This item is completely replaceable, but the health, mana regen and CD reduction for you and your team is fun. Generally, a match should be over before you even get to this point, but you never know.

*Note: The last 3 items are completely situational. I picked what I did because that's generally what my team needs from me. Kayle can be the most aggressive healer support ever because of the large amount of health and the big damage from her Q. She's got utility like none other, so I've built items to support that. With this build she can stay in the fight, fight hard and usually win the fight for her team.

*Note: A good replacement for the Soul Shroud might be something like Nashor's Tooth to help you get a little more burst under your belt.

Pros:
Good Healing
Able to jump around the map to save team mates
Very aggressive healer, can dps with her team

Cons:
Weak at first
Cannot solo lane
Heavy mana use

Kayle healing is hard at first, you can't really make a good charge until you get your heal up to level 2 or 3. So just hang back, throw some heals to your laning partner and wait for an opportunity to start going in for kills with your partner. With your massive CD reduction, you'll be able to throw your invulnerability shield more than you know. It's the best move when you need to save ******ed team mates or avoid ganks. If your laning mate needs heals more often, or if your opponents are super aggressive or obsessed with ganking your lane, spend more points on your W early and don't worry so much about your other non-ultimate abilities. If you and your laning mate are pushing your enemy laners hard, spec into your slow more so you can keep pushing and maybe get a few kills yourself.

I already covered this earlier with my Masteries, but it is important to remember that Kayle needs to be able to jump around the map if she's going to play healer. You'll save team mates from countless deaths and piss off your enemies to no end if you can get from the bottom lane to the top in just a few seconds, and do it repeatedly.

Clarity is for you (and hopefully your team mates) to stay in the fight longer and push harder. Kayle will use a lot of mana keeping herself and her laning mate up at the beginning and a lot more keeping the team up in team fights when you start running around. Clarity is a must have because of your massive mana pool. Clarity should be off CD every time you need it.

Kayle is one of the best team oriented champions. She can push everyone on her team, doing burst damage and healing all at once. She can pull team members away from the icy hand of death with speed boosts/enemy slows, invulnerability and heals. She's not good by herself. Although some situations may come up when you can steal a kill or two away from the group, you want to stay with your group most of the time. Quick/big heals and overall good utility will make Kayle a prime choice for a team in need of a healer that can stay in the fight.

This build brings Kayle's healing and support abilities to the forefront. If you want to do max dps as Kayle, I'd look into a different build. You can hit hard with this build, but you can't really sustain it or chase well. This build is for pushing your team forward with good heals and good offensive suppression.

Kayle is a great healer who can hit like a truck, heal like a boss and take hits like a tank. If you need support that can hang in there when big damage is going out, Kayle is what you want and this build will help you get her there.