Metronomes come in many different shapes and sizes. Some tap, others chirp, and still others speak! Metronomes can be set to specific speeds in order to tap at that speed. Then you, the violinist, use it to set the speed of the music you're playing. They're a great practice tool and a must for anyone wishing to truly focus on learning violin.
I'm going to focus on metronomes that would be best for a beginning violinist. There are metronomes out there that seem to require you to have a college degree to operate; you don't need anything that fancy!
When choosing a metronome, pay attention to the sound that they make. Some of the cheaper metronomes have problems with making an annoying high-pitched chirp that gets on your nerves after awhile. Others do not tick (or thump or chirp) loudly enough to be useful. Most of them have a headphone jack, but it's hard to play a violin with headphones on sometimes.
You also want to pay attention to the size of the metronome. I have a beautiful metronome that I've used for years, but it's rather large. This makes it difficult to bring with me since it doesn't fit in many violin cases. However it can count faster notes for me (such as eighth notes), change sounds (I love when it counts in its little tinny voice), and even can play tones for me to tune to.
Here are my metronome reviews. There are other good metronomes out there, but I've had good luck with these in particular.
This little palm-sized metronome is loud enough to be heard over your playing (which is more than I can say for some of the tiny cheap metronomes that make teeny chirp noises). You can plug in an earpiece to hear the beat better. It's a simple little device that lets you set the speed via a dial.
This small metronome has two different click sounds, which is good depending on if you're practicing higher or lower notes. It also has a volume control, which the Wittner metronome above does not. It gives you the Italian words for common speed markings in music, such as "Andante" and "Presto" so you have an idea of how fast the beat would sound when setting the dial there.
This metronome is not digital, it's mechanical. You wind it up using a little key and it ticks until it winds down. These tend to be very precise, despite what you may think. Metronomes have been made like this since the early 19th century. Can you imagine how classy this would look in your practice niche?
Like the metronome just above, it has musical speed markings such as "Andante" written on the front. It comes in three different colors and has a solid feel in the hand. This is less portable than the metronomes above, so it's more suited to staying in your practice niche.
For the beginner, learning violin, watching the pendulum will be invaluable to helping you get a true feel for rhythm that a little blinking light just can't do. True, this metronome is more expensive, but it's still less than dinner out at a ritzy restaurant for the family.