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To begin our journey about what common
languages do and how they work together,

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let's begin by dissecting the languages
needed for a web application.

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On second thought, let's begin at the
drive-through.

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Imagine you're at a drive-through.

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You order some food through one of those
giant intercom menus.

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That order's sent to the kitchen where
they make the food.

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When the food is ready, it's put into a
bag and handed to you through the window.

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When you originally placed your order at
the intercom,

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it was like typing a URL into the browser.

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The URL is sent to a web server,

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the kitchen, which then produces what you
asked for.

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When the content is retrieved, aka your
food is ready, it's placed in a bag and

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handed through the window, or the front
end of the website.

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Just like a drive-through wouldn't put
just any order through the window when you

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drive up, it's the same for a website.

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This is called the client server model.

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The client makes a request to the server.

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And the server gives back the content to
the client.

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Once the page has loaded,

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from this point on when you send messages
through the front-end,

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you are sending messages to the back-end,
which then returns what you asked for.

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For example, maybe you forgot to ask for
ketchup, so

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you ask the nice lady at the front for
some ketchup.

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She turns around and asks the kitchen for

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a packet of ketchup and hands it to you,
and you walk away happy.

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You can see this when you go to the
Treehouse website.

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When I pull up my Treehouse home page, I
only see my information.

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Like the points I've accrued,

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my name, and the course I'm on have all
come from the server.

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But the layout and styling are going to
look the same, whether it's you or

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anyone else signing on to the Treehouse
site.

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The front end, or the drive-through window
if you will,

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uses images in the languages HTML, CSS,
and JavaScript.

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HTML and CSS are not programming languages
exactly.

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HTML is a markup language which builds the
structure of the website.

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And CSS is a styling language which tells
the browser how

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to make that structure look.

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Let's leave those languages aside for now.

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You'll learn a lot more about them later.

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The back-end, or

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the kitchen in our analogy, is made of
what are called server-side languages.

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And there are many different languages
like PHP, Ruby, Java, and

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Python to choose from.

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Whether you're programming for a website
or mobile app,

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you're going to be dealing with a lot of
the same process you just saw for the Web.

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One of the big differences when developing
mobile versus the Web is

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the number of languages you'll need to
use.

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If you're programming for a mobile android
device like the Moto 360 or

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Samsung Galaxy,

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you'll probably just be using Java to
program the whole application.

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If you wanted to build something for a
mobile iOS device, like an iPhone or

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an iPad, you'll be using Swift or
Objective-C.

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Hopefully now you have a better grasp of
what back-end and

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front-end languages are and how they work
together.

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In the next video we'll talk about the
similarities and differences between some

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of the back-end languages we discussed
like Ruby, Python, PHP, and Java.