Let's first look at where high resolution and low resolution artwork gets used, then we can get technical.
High Resolution artwork is used in the Print industry. Be it brochures, posters, signage or anything that needs to be clearly visible when it is printed. The file sizes are generally large. (ie, they take longer to email to someone.)
Low Resolution artwork is used for viewing on computer monitors and projector screens. These appear on websites, in e-mails, on television, or in computer based presentations. (Anything that will be viewed on a screen or a monitor). The file sizes are small. They need to load appear fast on a website/e-mail, or in the case of presentations - run smoothly off a computer without playback stutter or delays. Big files make your computer work harder and slow it down. And they make transferring data via internet cables take longer.
Now, we will look at what happens when you send someone a low resolution file for printing.
Firstly, the best analogy I can make is, imagine you draw a cute neat heart on a deflated balloon in a fibre-tip koki pen. Now, what does that heart look like when you blow the balloon up to full size? Not quite the same. The edges are fatter and fuzzy, it looks shoddy and unclear. Remember this, I bring it up later again...
Now let's see this in action:
Below is a low resolution image of my logo, intended for use on a website/e-mail at the size you are seeing it at now:
Looks fine right?
Now, if a designer gets e-mailed this exact logo to put into a brochure or an advert in a magazine, and it needs to be about 6cm wide (about 2,5 inches) when printed:
It will print like this (horrid):
BUT, (I hear you shout) THAT LOOKS MUCH LARGER THAN 6CM WIDE??
It does only on this screen, because SCREEN is measured in what is called PIXELS, and print is measured in Millimetres/cm/metres (or inches). If I imported this logo into my design program for a brochure, it will import at 6cm wide, and it will print at 6cm wide, but look as terrible as it does above!
For something intended to be viewed on a screen only, graphic designers hardly even take millimetre size into consideration, we only look at pixel size! ... For anything viewed on a screen, we work with WYSIWYG (What You See Is What You Get).
Now I will post the same logo, at the correct (high) resolution for printing at 6cm wide:
Viola, that looks better right!? It will print clearly now!
So what is the difference between the clear logo and the fuzzy logo, why did that happen?
We get back to pixels and get technical now:
Pixels are measured per square inch. Industry standard. Pixels are tiny little blocks of colour that make up an image or photo.
The abbreviation in the industry is DPI (Dots Per Inch).
Low, or screen Resolution is 72 DPI (pixels per inch.)
High, or print Resolution is 300 DPI (pixels per inch.)
The original logo - Exhibit A was exported out of my design program, I set it to export at a low screen resolution of 72 DPI.
I set the width to 176 PIXELS wide (forget the height for now, it scales in perspective). The print size was 6cm wide, however at the time, that was of no consequence for placing it on the site as you see it, I wanted it 176 PIXELS wide, I was not interested in the print size. It had to look good on the screen at the size I wanted you to see it. (Effectively, I drew the heart on the balloon, with NO intention of blowing it up, ever!)
For Exibit B, I simply took Exhibit A, and scaled it up to a print resolution of 300 DPI. This automatically pushed the PIXEL size up to 709 PIXELS wide, and kept the print size at 6cm wide. It compensated from having 72 tiny dots of colour per square inch, to having 300 dots of colour per squre inch, by adding in its own dots of colour to fill up the extra space! That is why it is fuzzy and unclear. (I drew the heart on the uninflated balloon, and then blew it up!)
This pictured on the left, believe it or not, is zoomed in on the left top arm of the "H" of Headzone. This is how it compensated for the extra pixels it had to "make up" to increase the resolution from 72 tp 300 pixels per inch
On Exhibit C, the clear large one, I went back to my design program, and exported it for printing at 300 DPI from scratch. It is still 709 PIXELS in width, but in this case, the pixel size doesn't matter, I want a clear clean printed 6cm wide logo. (In this case, I blew up the balloon FIRST, then drew the heart on it nice and neatly while it was inflated!) This picture on the left shows how different and how much cleaner the top edge of the H is for printing.
So that in a nutshell is how important Resolutions is! The exact same thing happens with photos! 'Ribbet' the bullfrog below:
Here's Ribbet meant for a website avatar sized photo at low resolution at the size he is intended:
Ribbet blown up from the photo above to be used in print at 5cm wide (blehgg..):
Ribbet properly saved at the right size and resolution of 300dpi for a perfectly clear print at 5cm wide (as he should be!):