It is Bermuda grass. Read the Bermuda Bible.

Interesting. Are there wider blade varieties of Bermuda? I thought Bermuda had a narrower blade than what I have.

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Looks like centipede to me.

Centipede and St Augustine look much the same. Find a blade that has not been mowed. At the tip centipede will be pointed while St Augustine will have a boat tail shape.

But it has to be St Aug because centipede only grows in full sun on p-poor soil.

Best thing you can do is never let it die. While that may seem obvious, most grasses will go dormant when dry but St Aug goes straight to dead. That does not mean it has to be soaking wet all the time. It just needs to not dry out continuously for several weeks. Proper watering for St Aug is like all lawns. Water it deeply (one inch at a time) and infrequently (this time of year it might be every 14 days).

Mulch mow at your mower's highest setting for spectacular turf. It will take several weeks after you fertilize for all the grass to get up to that height but you will thank yourself for doing it. Tall St Aug will go much longer into a drought before it needs water. It will also help keep weeds out. I have a rural lawn surrounded by all kinds of weeds but none are sprouting up in the middle where it is tall. Now I'm working on spreading the grass out to mingle with the weeds and choke them out.

If you want it to excel, it really needs fertilizer like yesterday. You'll get the fastest results from using something organic like alfalfa pellets (rabbit chow) once a month at a rate of 20-30 pounds per 1,000 square feet. The reason that is faster than chemical fertilizers is that you can only apply chems once now and again in the fall or you risk burning the grass. Or you could apply both now and continue with the alfalfa for a few months to keep the green going. It will become much more dense, also.

Now for some pictures to illustrate what I'm talking about.

1. Tall St Augustine in a lawn setting. This is mowed at the mower's highest setting. Fertilizer is corn gluten meal.
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2. St Augustine that has not been mowed since last October. Pic was taken on Valentines Day when all the rest of the lawns were dormant but this one was not. Dog is 35 pounds so not a puppy. This is an experiment I'm doing with my rural lawn. Note there are no weeds in sight.
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3. Effect of alfalfa pellets on zoysia lawn. This lawn is just like yours (thin). The spot was fertilized in May 2011 and photo taken in June. Note the improved density, color, and growth.
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Thanks for the info from those who've posted. I have to say, though, I'm now more confused than ever.... Bermuda, centipede, St. Augustine!?!?!?! I figured some new pics might help (especially with blade tips as dchall mentioned).

I should also point out that the shadier parts of my lawn are NOT thriving. This leads me to wonder if I have Centipede, as the really thin areas in the photos are in shadier spots. These areas have stolons that area easier to see though (for photos), since there is less growth around.

Here they are (I added a quarter to photos as a reference):
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I don't know anymore.

In the last post, the first pic looks like st. augustine. And the second pic looks like SA on the left, and centipede on the right......

Now that we have some scale and close ups, no question it is SA. As Dave point out the tips of the blades is the give away, also the purple stolon.

Thanks for the additional info. After reading grasshole's post, I sent my pics to my local MSU extension office. They said it looked like a mix of SA and Centipede. However, from TW's post, it sounds like there MAY be a higher percentage of SA.

In any case, does this mean I need to choose one type to care for and maintain? Since both have very different care, it seems like I should start mowing higher and put down some fertilizer (for SA). Or, if I want to promote the centipede, mow low and no fertilizer.

Suggestions?

Care for per St Augustine needs... Likely someone tried to overseed with Centipede rather than fertilizing for St Augustine...

It will spread like crazy and fill in the bare spots...

Thanks

In any case, does this mean I need to choose one type to care for and maintain?
Yes you need to decide. Centipede and SA are not compatible. If I were you I would do a soil test and look at the PH to decide. If PH is 6 or higher SA wins. If lower than 6 Centipede wins.

Excellent photos! Thanks

First picture second set: You have a fungus in there. All those gray blades should be green. All the green blades with brown spots should be all green. This is what relatively healthy St Aug should look like. This spot happens to be a little yellowish, but at least it's not diseased.

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The grass in that picture was just rebounding from years of starvation. It was still a little thin as you can see some live oak leaves in the mix.