Liquid & Dry Malt Extract
LME or Liquid Malt Extract is the dehydrated liquid (wort) that looks similar to a molasses like syrup. The wort is produced from a typical grain mash, and the LME contains approximately 20% water.
DME or Dry Malt Extract is put through a second dehydration process that reduces the water to about 2%. If you are looking for a malt extract with a longer shelf life and easier measuring then you should lean towards the dry malt extract. When using DME it must be sealed in an airtight environment or it will start to absorb moisture and turn into a big lump.
Both types of malt extracts have their benefits, so be sure you are selecting the one that makes sense for your brew setup.