Okay, this podcast has an interview with one of the researchers involved in that announcement. TL;DW version: NASA found some weird mineral clumps on some rocks. Now, the only way on Earth that those kind of mineral clumps can form is either in conditions similar to those you would find around volcanic vents in the ocean, or in concretions made by bacteria. There's absolutely nothing about the area of Mars where those clumps were found that would indicate there was an underwater volcanic vent, so that certainly seems to point to there being at least bacteria on Mars at some point.
