Ice is Important:
Cocktails and mixed drinks would be no where without ice. Think about it - this is the one ingredient universal to almost every cocktail made (with the obvious exception of hot drinks and a few cocktails like the Champagne Cocktail). Ice not only chills drinks , but as it melts or is shaken it becomes a part of the mix and because of this, the frozen water deserves more than a little attention.
Forms of Ice:
There are four basic types, or forms, of ice (cube, cracked, shaved and block) and each have their uses. In Imbibe! David Wondrich quotes Jerry Thomas' 19th Century rules for using each, and these are still somewhat relevant in modern mixology. Thomas says: "As a general rule, shave ice should be used when spirits form the principal ingredient of the drink, and no water is employed. When eggs, mild, wine, vermouth, seltzer or other mineral waters are used...it is better to use small lumps of ice..." This is still sound advice but let's break it down for modern ice forms.
Ice cubes are good for almost all mixing: for shaking, stirring, drinks on the rocks, or with juices and sodas. The larger, thicker surface area makes a cube melt slowly and causes less dilution and it is customary to fill a glass or shaker 2/3 full for best results.
With the help of a Lewis bag, similar canvas sack or a clean towel, cubes can also be pounded into cracked or crushed pieces. The only other thing you need for this is a blunt object (ie. hammer, mallet, muddler) and some unwanted frustration that needs to get out. It's a little bit of work but quite therapeutic.
Smaller than cubes, cracked ice melts faster and adds more water to drinks. Usually this is used when making frozen drinks because cubes can clog blender blade and be inconsistent in the end. Two-thirds to one cup of cracked ice is perfect for a single frozen Daiquiri or Margarita. Typically bagged ice from the store is cracked.
Crushed or shaved ice is what you typically find in fountain soda machines. This is a very fine ice that can be used in a shaker to produce a thick, slurry of a cocktail. You can also use it to make an "adult snow cone" of sorts by packing shaved ice in a glass (or paper cone if you want to get "authentic") and pouring liqueurs over the top. Spirits like Chambord, PAMA and amaretto are great alone or you can build a custom flavor by combining a few.
Back in the day all ice bartenders used started as a block and it was up to the individual and their ice tools to create smaller, usable chunks and shavings for mixing. Luckily, we don't have to use picks and shavers anymore.
Today blocks are primarily used for chilling party punches and can take any form you want. Rings are popular and there are many novelty molds available but you can also use almost any container you have available as long as you can remove the solid ice.
Another large chunk of ice that is becoming more popular is the ice ball, which is commonly used in Japan for serving "whiskey on the rocks." Learn more about the ice ball and how to make one.
Making the Best Ice:
Basic science says that ice is water in a solid form and given that, it only stands to reason that cleaner water produces cleaner ice, which will add water to your cocktails in the end. Start off right by freezing water that you would drink: distilled, purified, natural spring or bottled, essentially anything but unfiltered tap water.
Keep your ice fresh by rotating the newer and older cubed. Avoid storing it in the freezer near foods like fish or anything else you don't want to taste in your next Highball.
At home ice cube trays are perfect for the nightly round of drinks. I keep a few trays full at any given time and have a regular rotation to keep all of them fresh. One-inch cube trays are ideal.
Many refrigerators are equipped with an ice maker which produces cubed, cracked or even shaved ice and are very convenient. If you rely on this and your cubes are caught in a bucket or tray automatically, be sure to rotate the ice so you have the fresher cubes available on top. Also, you may want to consider the extra energy an internal ice maker uses (see Eco-Friendly Bartending for tips).
Ice can be stored for easy access during a party in an ice bucket. These are usually insulated and will keep ice from melting quickly.
If you like, you may also want to infuse your ice. This is a simple way to add subtle amounts of flavor to drinks. One example is the rosemary-infused ice used in this Rosemary Lemonade recipe.
For an extensive and amazingly detailed study on ice, visit Camper English's Ice Series on Alcademics.