Ask any bartender and they will tell you that the most important ingredient in any cocktail that they create is ice. It is so important in fact, that it has been said that "ice is to a bartender, what fire is to a chef". It sounds a bit farfetched, but ice is extremely important in the creating and serving of cocktails, and there is much more to frozen water than first meets the eye.
The Role of Ice
Ice serves 2 main roles in cocktails; the first is in the creation, and the second is for serving. Ice is used for all methods of cocktail creation, including shaking, stirring and blending, and it's purpose in mixing is to chill and dilute the drink. When serving, ice plays the role of continuing to cool the drink. For both shaking and serving, you need to use full ice in the glass or mixing tin. This isn't to save money for the bar, but it is to minimise the dilution in the drink. If you really don't believe me, have a look at the second Law of Thermodynamics...
What kind of Ice?
The type of ice you're using depends entirely on what drink you are making, but before we get on to that you need to ensure that whichever ice you are using is 'dry'. I know it sounds silly, and I don't mean dry ice. Making sure it is 'dry' means ensuring that it isn't already melting before using it, ideally it should be straight out of the freezer or ice machine, and should be dry to the touch with no droplets of water forming. This is simply due to the fact that dry ice will take longer to melt, therefore it won't dilute your drink as quickly.
There are 3 types of ice you will commonly encounter: Cubes, Crushed and Blocks. Cubed ice is your standard ice and is used for shaking, stirring, blending and building cocktails. It is also used in drinks that require blending or rolling. It will dilute drinks at a medium speed, dry ice diluting it slowly.
Crushed ice is most commonly used in Mojitos, Juleps and drinks such as a Bramble. The idea is to chill the drink as much as possible, and also add quite a lot of dilution to the cocktail. Ice Blocks are either much larger cubes or spheres of ice. They are typically about 3/4 times the size of your normal cube, and are most commonly used in the serving glass. The size of the cube means that it retains its temperature for longer, meaning it stays cold and does not melt. You stir cocktails mainly to control dilution and to chill the drink, so when serving the drink you still want to minimise the dilution. It makes sense then to use ice which will take longer to melt!