What is "vector art"?
If you come to us with a request for, say, a t-shirt, it won't be long before we start talking about vector art. Vector art is created on a computer using (usually) Adobe Illustrator or (less typically) Corel Draw—a friend of mine worked in a sign shop and Corel was the preferred software app of that industry, but most every graphic designer I know uses Adobe Illustrator exclusively, it's the standard. Vector art is basically made of points, lines and shapes—you select the pen tool, click to make a point, then click again somewhere else on the page to make a line. Click a few more times in a few more places, then click back on the original point to close it up, and now you have a vector shape. It's like digital construction paper—once you have a bunch of shapes, you combine them, move 'em around, bring some to the front and send others to the back, and before you know it, you have an illustration.
We prefer to use vector art because a. it has nice clean lines, and b. you can scale it up and down without degradation or loss of resolution. The alternative to vector art is raster art, which is computer art that is made up of pixels—tiny squares. Resolution is a measure of how many of those tiny squares are in an area—say, an inch. If your resolution is 300 ppi (pixels per inch), that means literally, there are 300 little pixels in a 1" x 1" square. 300 is considered fairly high resolution. However, if your resolution is 72 ppi (you may hear dpi—dots per inch—sometimes too, it means the same thing, even if it's not as accurate) you will only have 72 pixels in that same square, so the pixels will naturally be larger and the picture won't look as good. Here, take a look: the picture below is a digital image of me on vacation last summer, looking pensive:
If you zoom in, you can start to see the pixels.
And if you zoom WAY in—this is at 3200% magnification—you can really see those pixels at work.
Another issue with raster images is they are generally much larger (in terms of file size) than vector images, so to mitigate that, file formats often incorporate compression—throwing out pixels they consider unnecessary to give you a smaller file size. Ultimately, you can get images that appear blurred or jaggy; compressed file formats include .tif and .jpg (and in a different way, .gif and .png). Facebook compressess every image you post, to some degree, so if you ever wondered why your Facebook photos look crummy, now you know.
Vector images, on the other hand, look great no matter what size you make them. Resolution isn't an issue, because there are no pixels. Take a look at this cartoon I drew of my friend Tug:
Looks neat. But when you zoom in, look at what happens:
That's right: nothing happens, it still looks great! And even though there's a brush stroke on his facial hair (to make it look scruffy), you can see when you zoom in that even the brush strokes are smooth. We could make this 100' tall or 1" tall and it would look just as sharp. That's the power of vector art! Vector images are often used for illustrations, company logos and yes, screen printing and apparel. File formats will usually end in .ai or .eps, but if you have a PDF that was originally created as vector art, that could work as well.
So there you have it—consider yourself a vector master!