With any instrument, there are always times that you want to play it as loud as possible, but also times that call for a softer approach. In the violin's case, those times generally fall into two specific scenarios, and, accordingly, there are two kinds of violin mute to choose from. The first type is the practice (sometimes called the hotel) mute, which has a very strong dampening effect and is meant for lowering the violin's volume enough for you to practice near others without disturbing them.
The second kind of mute is the orchestral mute, used during performances to reduce the overtones produced by your strings. This will give the violin a mellower, smoother sound and is often called for in music by the direction "con sordina." Some examples of practice mutes are the Glaesel Violin Ultra Practice Mute and The String Centre Metal Practice Mute. These fit over the top of the bridge, making them quick and easy to install and remove. As a rule of thumb, harder materials provide greater sound attenuation. So, if you're simply looking to avoid bothering the next-door neighbors on your street, a soft rubber mute is more than enough. On the other hand, if you have family in the same room and you need the lowest volume possible, a metal practice mute is going to be the best option. You might even decide to collect a range of practice mutes so you can pick the best one for the volume level you need at any given time. Unlike practice mutes, orchestral mutes are usually designed to fit on the strings so that you can slide them on and off of the bridge, easily muting and unmuting the violin as required by the piece you're playing. One of the most popular orchestral mutes is the Tourte Single Hole model. This can be placed on the A string, or suspended between the A and D strings.
Some other great options are the two-hole tourte style mutes, and the Super Sensitive Spector Violin Mute, designed by Fred Spector of the Chicago Symphony Orchestra. Whichever type of violin mute is right for you, there is a great selection here to choose from. Every violinist should absolutely have at least one practice mute in their collection, as well as their orchestral mutes of choice. They'll give you the versatility to practice anywhere, and to change the timbre of your violin when the sheet music calls for it. That's an important part of your musician's toolkit, so don't be shy about gearing up with a good range of mutes.