Malt extract is a concentrated, unhopped wort. It is produced and concentrated by the maltster.
When homebrewing with malt extract, you dilute it with water and then boil it with hops added at different times during the boil (hop additions), cool down and add yeast. Hops and yeast are purchased separately.
Malt extract is also suitable for baking and cooking.
The main difference between liquid and dried malt extract is the amount of liquid left in the extract. Liquid malt extract (malt syrup) contains about 20% water and 80% fermentable and unfermentable sugars. Dried malt extract or malt spray contains virtually no water at all.
1 kg of dried malt extract is equivalent to about 1.2 kg of liquid malt extract, which is enough for approximately 8 liters of normal wort (OG 1045).