This means they can look for tens of thousands of DNA sequences all at once. A sequencing array is made of many different short DNA sequences. A researcher chops the unknown sequence into short bits, sees where the bits bind on the array, deduces the sequences of all the short unknown bits, then reassembles the overlapping sequences into one long sequence. An expression array is made up of many different long DNA sequences, each complementary to every mRNA sequence that a certain cell can make. A researcher breaks a cell preparation open, extracts all the mRNA sequences it contains at that moment, and puts those on the expression array to see which ones are there.
Researchers love DNA chips because they give a huge amount of information, fast, at low cost.
When the price comes down enough, microarrays will likely become routine tools in the doctor's office.