A Block is a Master Pattern, usually made of a thin cardboard, which you use (by tracing around or marking through with a pinwheel) to make the pattern on thin paper, which is then cut out and pinned to fabric.
A pattern is a finished design ready for cutting out and sewing.
As blocks are made from cardboard, it is easy to trace around them. You usually make design-line markings on the blocks which you then transfer to the paper pattern, and you can also refer to those design lines on the block when making future patterns with that same Block. Blocks are usually stored by hanging them on a pattern hook, they aren't folded up like paper patterns.
The image below shows a set of Blocks I purchased for the TAFE Patternmaking course, the whole block set is hanging up on two pattern hooks. All the Blocks in this set are Women's Size 10 and consists of the following pieces: (Bodice + corresponding Sleeve), (Blouse + Sleeve), Skirt, Pants, Jeans, (Extended Line Dress + Sleeve), (Jacket + Sleeve).
In a Nutshell
If it's made of cardboard, and you use it as a Master to make other patterns, it's a block. If it's made of tissue or other paper, meant to be placed onto fabric for cutting out, it's a pattern.