Tea, including both green tea, and regular black tea (what most people mean when they just say "tea") is pretty good for you, much in the same way other natural foods, like fresh fruits and vegetables are. But it's not a miracle drug, and people often over-hype it, especially green tea.
There's not much evidence supporting the idea that green tea is healthier than other teas.
Tea also contains caffeine, and other substances that aren't great for you if you have too much of it. Like anything, it's great in moderation (3-5 cups of tea a day are fine) but it's not good to have too much of it.