Yeast will have a huge impact on a beer!

Without the yeast it will not be a beer, just a sugary brown liquid called wort. The yeast takes over from the mash enzymes and converts the sugars that they produced from the malt starches into alcohol and at the same time adding taste and character to the beer. There are two categories of beer yeasts, top-fermenting ale yeasts and bottom-fermenting lager yeasts.
Yeast for brewers are sold in two different forms, dry and liquid. The dry yeast is the more convenient, cheaper alternative while the liquid yeast is more expensive and offers a wider variety of styles.

As the dry yeasts are easier to use I tend to save the liquid yeasts for more advanced brews and go for the dry ones for my ordinary beers. The Nottingham dry yeast from Danstar is my most commonly used one but I like the range of yeasts from Fermentis as well, like the Safale S-04, US-05, T-58 and the S-33.
Some of the liquid yeasts I have used includes the Wyeast range: the 1084 Irish Ale, the 1099 Whitbread Ale and the 1318 London Ale III.