Our Sun is one of the calmer stars in the known universe - especially when compared to some of the giants out there which routinely emit gigantic solar flares. But even our Sun will grow to become a threat to us one day.
To give it a cosmic classification, our Sun is currently a yellow dwarf star. As it approaches the end of its natural life and is no longer able to carry out fusion, it will eventually become a white dwarf. Before it reaches that stage, though, it will briefly (in universal terms) become a red giant. That won't be good for Earth, because as a red giant, the Sun will expand to a size where it reaches our orbit and engulfs us. Not that anybody will be left to see it, because the rapid expansion of the star would have long since burned everyone to death. On the bright side, if you'll excuse the pun, we have a long time to come up with a survival plan. The estimated life of our Sun is 10 billion years, and we're only somewhere around half the way through it.