Sugars; such as Dextrose and White Sugar will increase alcohol content, thin a beer out and is often to blame for 'cidery' tatses in beer. Although recommended by Kit producers it is not an ideal option. These are the same people who give you half as much yeast as needed and tell you to ferment in the mid to high 20's. Only really needed in small amounts, 100-300gm to help dry out a beer and/or boost ABV. Try to avoid.
DME or LDM, is a powder produced from Malted Barley as makes beer, in a usable, easy to store product. Readily available and a much better option in kit beers than plain sugars. Will aid body, alc and flavour.
LME is beer wort reduced to a concentrate, the same as your kit brew but without hops/hop extract added. An excellent addition to any kit beer and can be used as a base for extract brewing, it is less concentrated than LDM and you will need about 10% more by weight to get the same result.
When I brewed with kits I found the best results came from using 1 or 2 packs of LDM(.5 to 1 kg) and about 1 cup of Dextrose. Varying the LDM depending on the end alc volume you want.
Look for Ian's Kit and Extarct Spreadsheet; it is very useful for calculating your malt and sugar requirements.
