High tech refers to technology that is at the cutting-edge—the most advanced technology currently available. The noun form is hyphenated: high-tech or high-technology. There is also a style of architecture known "High Tech".
There is no specific class of technology that is high tech—the definition shifts over time—so products hyped as high tech in the 1960s would now be considered, if not exactly low tech, then at least somewhat primitive. This fuzzy definition has led to marketing departments describing nearly all new products as high tech.