Yeast come in two main product forms, dry and liquid. (There is also another form, available as pure cultures on petri dishes or slants, but it is generally used as one would use liquid yeast.) Dry yeast are select, hardy strains that have been dehydrated for storability. There are a lot of yeast cells in a typical 7 gram packet. For best results, it needs to be re-hydrated before it is pitched. For the first-time brewer, a dry ale yeast is highly recommended.
Dry yeast is convenient for the beginning brewer because the packets provide a lot of viable yeast cells, they can be stored for extended periods of time and they can be prepared quickly on brewing day. It is common to use one or two packets (7 - 14 grams) of dried yeast for a typical five gallon batch. This amount of yeast, when properly re-hydrated, provides enough active yeast cells to ensure a strong fermentation. Dry yeast can be stored for extended periods (preferably in the refrigerator) but the packets do degrade with time. This is one of the pitfalls with brewing from the no-name yeast packets taped to the top of a can of malt extract. They are probably more than a year old and may not be very viable. It is better to buy another packet or three of a reputable brewer's yeast that has been kept in the refrigerator at the brewshop. Some leading and reliable brands of dry yeast are DCL Yeast, Yeast Labs (marketed by G.W. Kent, produced by Lallemand of Canada), Cooper's, DanStar (produced byLallemand), Munton & Fison and Edme.
Dry yeasts are good but the rigor of the dehydration process limits the number of different ale strains that are available and in the case of dry lager yeast, eliminates them almost entirely. A few dry lager yeasts do exist, but popular opinion is that they behave more like ale yeasts than lager. DCL Yeast markets two strains of dry lager yeast, Saflager S-189 and S-23, though only S-23 is currently available in a homebrewing size. The recommended fermentation temperature is 48-59°F. I would advise you to use two packets per 5 gallon batch to be assured of a good pitching rate.
The only thing missing with dry yeast is real individuality, which is where liquid yeasts come in. Many more different strains of yeast are available in liquid form than in dry.
Liquid yeast used to come in 50 ml foil pouches, and did not contain as many yeast cells as in the dry packets. The yeast in these packages needed to be grown in a starter wort to bring the cell counts up to a more useful level. In the past few years, larger 175 ml pouches (Wyeast Labs) and ready-to-pitch tubes (White Labs) have become the most popular forms of liquid yeast packaging and contain enough viable cells to ferment a five gallon batch.