Adobe Illustrator is a software for professional designers and is intended to create and edit vector graphics. Unlike Photoshop, Illustrator can create, open or edit vector scalable graphic files (.eps, .svg) that are not loosing quality when dimensions are changed. While files made in pixels have some limitations, a vector file can be used in any dimension (for example in big posters, printing materials, logo etc) without loosing quality.
If you plan to create graphics to be used in different media, including print, Illustrator is the recommended choice, instead a pixel based image editor.
The program was released first time in 1987 and since then is the most used commercial software by designers to create vector graphics.