Different extracts have different degrees of fermentability. In general, the darker the extract, the more complex sugars it will contain and the less fermentable it will be. Amber extract will typically have a higher finishing gravity than pale extract and dark will be higher than amber. This is not always the case, though. By manipulating the mash conditions, the relative percentages of sugars that are extracted from the mash can be varied. A brewer can produce a wort that is almost entirely made up of highly fermentable sugars like maltose or he can produce one that has a higher percentage of unfermentable complex carbohydrates. Because these complex sugars are not very fermentable, the beer will have a higher finishing gravity. While most of the perception of a beer's body is due to medium length proteins, the unfermentable complex sugars will lend some of the same feel.
For example, Laaglander'sú DME from the Netherlands is a high quality extract that often has a finishing gravity as high as 1.020 from a common 1.040 OG. The heavier body is nice to have in a stout for example; all-grain brewers would add American Carapils malt (a.k.a. Dextrin Malt) to their mash to produce the same effect. Brewers using extract have the alternative of adding Malto-Dextrin powder, which is a concentrated form. Malto-Dextrin powder has no taste, i.e. it's not sweet, and is slow to dissolve. It contributes about 40 points per pound per gallon.
Typical Sugar Profile Extracted From Malted Barley
Other Complex Carbohydrates including Dextrins
To summarize - malt extract is not some mysterious substance but simply a concentrated wort, ready for brewing. You don't need to agonize over which kit to buy, comparing labels and product claims; you can plan your own beer and buy the type of extract that you want to use to make it. Malt extract makes brewing easier by taking the work out of producing the wort. This lets a new brewer focus on fermentation processes.
The biggest step for a homebrewer is to learn how to extract the sugars from the malted grain himself. This process, called mashing, allows the brewer more control in producing the wort. This type of homebrewing is referred to as all-grain brewing, because the wort is produced from the grain without using any malt extract, and it won't be discussed until Section 3 - Brewing Your First All-Grain Beer. In Section 2 - Brewing Your First Extract-and-Steeped-Grain Beer, we will examine the middle ground of this transition and take advantage of the benefits of grain with less equipment. You can use steeped specialty grains to increase the complexity of extract-based beers, and you will probably want to try it for your second or third batch, but it is certainly not difficult and could be done for a first beer.