One of most difficult and misunderstood aspects of playing the violin is learning when and how to use a glissando, to slide from one note to another. This is a somewhat mystic art form itself, and sort of a musician's Kabbalah. Nothing can make listening more pleasurable than if the instrumentalist knows when to a use a tasteful glissando, and how to execute one to perfection. Even more important is to know when not to attempt to do one. The French school was responsible for developing rules for different types of glissandi, although the first master and founding father of beautiful playing was Austrian-born Fritz Kreisler. He was also the first famous violinist to use continuous vibrato but more on that another time.
There is such a thing as too much of a good thing. The slow movement of the J.S. Bach Concerto for Two Violins was at some point recorded by Kreisler and Efrem Zimbalist. They tried to outdo each other in the glissando department and the result sounds like a pair of vocal male cats in March having a go at it. But all in all, glissandi can add a wonderful personal touch, a perfect spice to bland food. If you listen to an old recording of Kreisler playing the Albeniz Tango, tears will roll down your face if you have any sensitivity left. Then listen to Jacques Thibaud play the same piece and you are astonished to find how differently it is executed but equally magnificently. In fact you soon learn to tell the two masters apart so easily you'll have no trouble knowing who is playing if you hear their playing in a random order. You won't be as successful with present day Juilliard violinists produced in an assembly line!
The violin, ideally, is a instrumental replacement for the human voice. If you can get your hands on duo recordings by Kreisler and the famed tenor John McCormack you'll instantly realize and appreciate this kinship. Unfortunately in both fields the art has been declining for a long time. Much of this can blamed on teachers who themselves were poorly taught and often were bad players or singers to start with, unable to demonstrate things. 30 seconds of showing how for a gifted student is worth more than thousands of words. Much good is a violinist who knows how to play the instrument in theory but not in practice!
Part of reason glissandi came to be was the need to shift from one position to another, usually in order to keep a melody or other figure on one or two strings. Every string has a totally different color and the beauty of a musical line is often ruined by careless crossing to a brighter string, just because it is easier to do so. However, every shift need not be a slide, quite the contrary. A glissando is after all an embellishment that should be used sparingly and with great care.
In orchestral playing a glissando falls into the category of special effects. A seemingly good idea can and often does produce horrific results. Once in a while a composer marks a line from one note to another, indicating portamento, but this didn't happen in music before Mahler's time.
My advice to fellow musicians: learn from the past, and honor and admire what is now available on historical recordings. Of course styles have changed as fashion always does, but the basic truth is out there. And to conductors: remember my comparison to the Kabbalah. This topic should be left alone unless you have spent a lifetime studying its mystic message and meaning.