user19126: Can you power the Arduino off a USB cable alone, without using the regular power connection? If so, what's the point of the additional power connection: to provide extra power?
Also Why not just have two usb ports one for power one for connecting to the computer, or a micro usb.

user18846: Arduino, in its very esence, is an embedded system and can be used as a standalone device. The USB port is provided for:

To power up the device.
To program it through a computer i.e. dump the sketch on it which one can program in the Arduino IDE.

Once you have dumped the program on it, it will stay on it forever unless you change/erase it and hence you can use the Arduino anywhere without actually bringing a computer (USB port) along.

user10214: As Hyperbola pointed out, the USB port function is primarily to allow for programming the board. Now, since the USB port has the power cables too, it is convenient to power the device from the USB port.
The main drawback of USB port powering, however, is that it can supply a limited current: usually the USB2 ports can deliver up to 500mA before shutting down. In order for the arduino board to be compliant with this, consequently, a 500mA fuse was put on the board just after the 5V usb input.
Now, what if your final circuit requires more than 500mA? Will you have to change board? Of course not! you can just use the external power input to power the arduino board and all the other circuitry without worrying about the 500mA limitation. Of course the onboard regulator has limits too, but you have more flexibility.
Moreover portable applications run on batteries. No battery can supply exactly 5V (usually alkaline ones deliver 1.5V per battery, while lithium have usually 3.7V/cell, but this varies over time). Consequently they put an onboard regulator for providing 5V from any 6.5V+ source.
Summarizing: the USB power is convenient, since you will usually power it from the PC when programming. An external connector, however, is very important because it gives you more flexibility when choosing the power supply. You don't have to use both: you can just choose the most suitable for your needs.

user19034: 
Can you power the Arduino off a USB cable alone, without using the
  regular power connection?

You can use the USB cable alone to power the board.

If so, what's the point of the additional power connection: to provide
  extra power?

You can use either to power the board. By using just the power plug you can make your Arduino portable by using a battery box or power it via a wall plug.

Also Why not just have two usb ports one for power one for connecting
  to the computer, or a micro usb.

Only ONE USB is needed to power and program.




19126: <p>Can you power the Arduino off a USB cable alone, without using the regular power connection? If so, what's the point of the additional power connection: to provide extra power?
Also Why not just have two usb ports one for power one for connecting to the computer, or a micro usb.</p>
 18846: <p>Arduino, in its very esence, is an embedded system and can be used as a standalone device. The USB port is provided for:</p>

<ol>
<li>To power up the device.</li>
<li>To <strong>program</strong> it through a computer i.e. dump the sketch on it which one can program in the Arduino IDE.</li>
</ol>

<p>Once you have dumped the program on it, it will stay on it forever unless you change/erase it and hence you can use the Arduino anywhere without actually bringing a computer (USB port) along.</p>
 10214: <p>As Hyperbola pointed out, the USB port function is primarily to allow for programming the board. Now, since the USB port has the power cables too, it is convenient to power the device from the USB port.</p>

<p>The main drawback of USB port powering, however, is that it can supply a limited current: usually the USB2 ports can deliver up to 500mA before shutting down. In order for the arduino board to be compliant with this, consequently, a 500mA fuse was put on the board just after the 5V usb input.</p>

<p>Now, what if your final circuit requires more than 500mA? Will you have to change board? Of course not! you can just use the external power input to power the arduino board and all the other circuitry without worrying about the 500mA limitation. Of course the onboard regulator has limits too, but you have more flexibility.</p>

<p>Moreover portable applications run on batteries. No battery can supply exactly 5V (usually alkaline ones deliver 1.5V per battery, while lithium have usually 3.7V/cell, but this varies over time). Consequently they put an onboard regulator for providing 5V from any 6.5V+ source.</p>

<p>Summarizing: the USB power is convenient, since you will usually power it from the PC when programming. An external connector, however, is very important because it gives you more flexibility when choosing the power supply. You don't have to use both: you can just choose the most suitable for your needs.</p>
 19034: <blockquote>
  <p>Can you power the Arduino off a USB cable alone, without using the
  regular power connection?</p>
</blockquote>

<p>You can use the USB cable alone to power the board.</p>

<blockquote>
  <p>If so, what's the point of the additional power connection: to provide
  extra power?</p>
</blockquote>

<p>You can use either to power the board. By using just the power plug you can make your Arduino portable by using a <a href="http://www.aliexpress.com/item/Battery-Case-with-Clip-for-4pcs-AA-battery-Arduino-Storage-Holder-Box-Case-Lead-FZ0975/32371855864.html?spm=2114.01010208.3.20.FsRn6V&amp;ws_ab_test=searchweb201556_9,searchweb201602_2_505_506_503_504_10032_10020_502_10001_10002_10017_10005_10006_10003_10021_10004_10022_10018_10019,searchweb201603_9&amp;btsid=5dfc0189-3cc3-4582-846c-0eb45294bea7" rel="nofollow noreferrer">battery box</a> or power it via a <a href="http://www.aliexpress.com/item/9V-1A-AC-DC-Power-Suppy-Adapter-WALL-WART-PLUG-Arduino/32614338731.html?spm=2114.01010208.3.2.LTo710&amp;ws_ab_test=searchweb201556_9,searchweb201602_2_505_506_503_504_10032_10020_502_10001_10002_10017_10005_10006_10003_10021_10004_10022_10018_10019,searchweb201603_9&amp;btsid=2d7049b6-4056-45c3-aeba-8d9667ab7e8a" rel="nofollow noreferrer">wall plug</a>.</p>

<blockquote>
  <p>Also Why not just have two usb ports one for power one for connecting
  to the computer, or a micro usb.</p>
</blockquote>

<p>Only ONE USB is needed to power and program.</p>

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