so are these decoders? or de multiplexers?
Well according to the datasheet for the 74HC138, they are "3-to-8 line decoder/demultiplexer; inverting"...in other words, this particular part can be used either need. There is also a complement part that does the 8-to-3 line encoding/multplexing (don't remember the part number off-hand, but it would be easy to find)...
