Procedural memories are 'implicit' because it's kind of ingrained. Like, with remembering how to ride a bike, it's difficult to explain how you know it, you just do. Declarative memories are 'explicit' because you can explain how you know it more easily.
Working memory is a difficult concept to grasp for some reason, but you should know anyway:
'Working memory' is a theory that describes the memory system between sensory memory and LTM, suggesting that memories in this stage have constant manipulation. The information we need to manipulate in our working memory will be either visual or auditory, and for both there is a subsystem involved in specifically manipulating that sense.
The visiospatial sketchpad is involved with visually manipulating information. For instance, you look at a map to where you're supposed to go, then look up at the road and point out this information to the driver.
A nice definition: The visuospatial sketchpad is assumed to hold information about what we see. It is used in the temporary storage and manipulation of spatial and visual information, such as remembering shapes and colours, or the location or speed of objects in space. It is also involved in tasks which involve planning of spatial movements, like planning one's way through a complex building.
The phonological loop deals with auditory information. It consists of two parts: a short-term phonological store with auditory memory traces that are subject to rapid decay and an articulatory rehearsal component that can revive the memory traces. The store is just like having any information in STM and not rehearsing it, the second store describes rehearsal or other manipulation.
The central executive coordinates these two systems and is involved in selective attention.