Adobe Illustrator is used for creating a variety of graphics you can produce logos, detailed illustrations, leaflets, web page designs and many more. So this is the big one, everything we create in Adobe Illustrator is a Vector which means there are no pixels involved at all. The graphics are made out of paths and anchor points which make them saleable to any size with no loss of quality.
This is why all companies should have a vector version of their logo, with the vector version you can put it on the side of a building or on your letterheads and you will see no loss of quality. There are other vector based programs on the market but Adobe Illustrator is the industry standard. As with all the apps in the Adobe Creative Cloud they work very well together, you can place your vector Adobe Illustrator files in to Adobe InDesign and swap between the two Apps very quickly and easily. You can also embed your Adobe Illustrator files in to an Adobe Photoshop layer and keep the vector qualities.
When creating layouts or stationary designs in Adobe Illustrator there will become a time when you want to visualise more than one page of a website or create an Adobe Illustrator file that has Letterheads, compliment slips and business cards all kept together. With Adobe Illustrator you can quickly and easily create a number of different sized art-boards all in the same document.