The transistor acts like a diode and?
Yes, the transistor acts like a diode. That is, essentially, what it does. What it also does, and what give it its added value and ability to amplify, is that the base current causes the collector-emitter "diode" junction to vary in its turn-on characteristic. With this ability, you can control a large current with a small current, and a small delta-current in the base causes a larger delta-current in the collector, the ratio being hFe, hence the term "gain".