What are MInecraft snapshots named after

The format is pretty simple. The first two digits you've rightly surmised to be the year the snapshot was released, in your example, 2018. The two digits following the "w" indicates which week of the year the snapshot was released (the "w" itself is just an indicator that the following two digits do represent the week number), and follows the ISO week date. In your example, that would be the 15th week of the year, or probably around the 9th to 14th of April. The trailing letter indicates how many snapshots there were for this week prior to this snapshot, "a" meaning "this is the first snapshot", "b" the second, and so on.