What is a Key/Value store database?

I've been looking at the wikipedia page for NoSQL and it lists several variations on the Key/Value store database, but I can't find any details on what it means by Key/Value store in this context. Could someone explain or link an explanation to me? Also, when would I use such a database?

Are you familiar with the concept of a Key/Value Pair? Presuming you're familiar with Java or C# this is in the language as a map/hash/datatable/KeyValuePair (the last is in the case of C#)

The way it works is demonstrated in this little sample chart:

Color        Red
Age          18
Size         Large
Name         Smith
Title        The Brown Dog

Where you have a key (left) and a value (right) ... notice it can be a string, int, or the like. Most KVP objects allow you to store any object on the right, because it's just a value.

Since you'll always have a unique key for a particular object that you want to return, you can just query the database for that unique key and get the results back from whichever node has the object (this is why it's good for distributed systems, since there's other things involved like polling for the first n nodes to return a value that match other nodes returns).

Now my example above is very simple, so here's a slightly better version of the KVP

user1923_color    Red
user1923_age      18
user3371_color    Blue
user4344_color    Brackish
user1923_height   6' 0"
user3371_age      34

So as you can see the simple key generation is to put "user" the userunique number, an underscore and the object. Again, this is a simple variation, but I think we begin to understand that so long as we can define the part on the left and have it be consistently formatted, that we can pull out the value.

Notice that there's no restriction on the key value (ok, there can be some limitations, such as text-only) or on the value property (there may be a size restriction) but so far I've not had really complex systems. Let's try and go a little further:

app_setting_width      450
user1923_color         Red
user1923_age           18
user3371_color         Blue
user4344_color         Brackish
user1923_height        6' 0"
user3371_age           34
error_msg_457          There is no file %1 here
error_message_1        There is no user with %1 name
1923_name              Jim
user1923_name          Jim Smith
user1923_lname         Smith
Application_Installed  true
log_errors             1
install_path           C:\Windows\System32\Restricted
ServerName             localhost
test                   test
test1                  test
test123                Brackish
devonly
wonderwoman
value                  key

You get the idea... all those would be stored in one massive "table" on the distributed nodes (there's math behind it all) and you would just ask the distributed system for the value you need by name.

At the very least, that's my understanding of how it all works. I may have a few things wrong, but that's the basics.

obligatory wikipedia link http://en.wikipedia.org/wiki/Associative_array

StackExchange Database Administrators Q#607, answer score: 42