Declarative knowledge involves knowing THAT something is the case - that J is the tenth letter of the alphabet, that Paris is the capital of France. Declarative knowledge is conscious; it can often be verbalized. Metalinguistic knowledge, or knowledge about a linguistic form, is declarative knowledge.
Procedural knowledge involves knowing HOW to do something - ride a bike, for example. We may not be able to explain how we do it. Procedural knowledge involves implicit learning, which a learner may not be aware of, and may involve being able to use a particular form to understand or produce language without necessarily being able to explain it.
Cognitive models of learning differ in the role they accord to these two types of knowledge: traditional information processing models, like Shiffrin and Schneider's dual processing theory, assume that declarative knowledge precedes procedural knowledge.
Emergentist models, on the other hand, claim that procedural knowledge can develop without a declarative stage, such that certain types of complex skills can be performed but not explained.
Schmidt's noticing hypothesis claims that learners may apperceive aspects of the target language which are then incorporated into interlanguage as procedural knowledge.
Read more: Awareness