Paint is a weird thing.  It seems so simple.  You buy a can and roll it on the wall, right?  Not really.  Once you start to look a little closer, you realize paint is in fact a vastly complicated subject.  
How do paints differ?You can buy a can of paint for 15 bucks.  You can buy a can for 70 bucks.  What's the difference?  Unless you think about paint all the time like me, you probably don't have much of an idea what you are paying for in that $70 can.  
Before I buy a can of paint, these are some of the things I might be thinking about: 
- How much area will a can cover?  - How well will it hide the color behind it?  - How does it smell?  - How well will it stick to the wall?  - How easy is it to apply? - How easy is it to clean up?  - How durable is it?  - How flexible is it?  - How well do the brush and roller marks level out?  - How well does it resist fading?  - How well does it resist moisture and mildew?  - What types of materials can it be applied to?  - How long does it take to dry?  - How safe is it?  - What sheens are available?  - How much does it cost?  - How much does it drip, sag, and spray off the roller?  - What kind of service will I get with it?  - Can it be easily touched up in the future?  
There is no such thing as a perfect paint.  As a general rule, of course, you get what you pay for.  But there are exceptions.  On a ceiling, for example, you aren't too worried about people touching it.  A less expensive paint will work fine and the super flat sheen of some cheaper paints can actually look better.  
Brands of paint:There are basically 3 brands of paint that I regularly run into: Sherwin-Williams, Benjamin Moore and Behr.  You may have heard of Glidden or Valspar or a few others, but those are my big 3.  Which one is the best?  There is no easy answer to that.  The thing many people don't realize is that within each of these brands there are numerous different lines of paint.  
Sometimes customers have very strong feelings about a particular brand of paint because of a bad experience.  For the most part, these opinions are unwarranted.  When I do a little digging and find the old cans in their basement, I usually discover that they used a good brand, but just a really cheap line of paint.  
That does not mean, however, that I do not have a preference.  
Home Depot's Behr paints perform well in tests, but they can be difficult to use.  These paints are heavily marketed toward the DIY market who will likely blame themselves if the paint job doesn't look great.  Behr has nothing to offer at the top end of the market.  Very few professionals use Home Depot paint.  
Benjamin Moore sells fine professional paint, but they are just obnoxious to deal with.  The stores are all independent.  Half the time there are just a bunch of dusty paint cans in the back of a hardware store.  Who knows what you're getting?  Employees are condescending, grumpy old men or biker dudes with tatoos.  
My favorite brand is Sherwin-Williams.  I prefer Sherwin-Williams for a few reasons.  Sherwin-Williams stores are all over the place.  They are convenient to pretty much everybody.  They are all clean and well organized.  They specialize in paint.  The people are always friendly and knowledgeable and they know my business.  They have color specialists who will come to your home.  They all have my account and all my information at all the stores.  They just have their act together much more than anybody else.  They are a pleasure to do business with.  
I like to stick with one kind of paint because I know I have a system that works.  When you change the paint, you introduce all kinds of weird variables that you may never see coming.  Maybe it will need more coats of paint.  Maybe it's lumpy.  Who knows?  
I remember one job where a customer insisted on Benjamin Moore's Regal Select paint.  Regal Select is a quality line of paint, so I didn't anticipate any problems.  I used the same roller cover I always do, but it just wasn't working.  I could only get paint on one side of the roller.  Instead of turning, it would just slide around in the pan!  
Painting with a different kind of paint is like switching to an Android from an iPhone.  It basically does most of the same stuff, but it will just be a little slow and awkward until you get used to it.  
ColorAvoid yellowAvoid colors that are too similarGet a pro consultationPaint a sampleDifferent light and time of day

Sheen & Gloss
names vary between lines and manufacturersflat paints are cheaperOn the ceiling?Play room?Bedrooms?Bathroom?

Paint as protection
What does a primer do?