All stars, no matter how large or small go through a lifecycle that at its most basic level is the same for all stars. Virtually every star in the universe starts off as a cloud of dust and gas in the InterStellar Medium (ISM). Some external force will eventually cause part (or parts) of this cloud (maybe a clump of slightly higher than average density) to start collapsing under its own gravity. As it does so, the gas and dust will become hotter and denser, thus attracting more dust and gas which will increase the mass, density and temperature of the clump, or protostar, still further. Eventually the core of this clump will reach temperatures sufficiently high that nuclear fusion of hydrogen atoms commences - at this point the star can be defined as being born.1.1 This new star will then continue to accrete most of the remaining dust and gas around it (some may or may not go into forming planets) and the star then lives out its life on the main sequence until it starts to run out of nuclear fuel (ie. has converted most of its initial hydrogen into heavier elements). At this point the star will swell up and through various different methods (that depend primarily on the initial mass of the star) will shed much of its outer section back into the ISM, thus forming new gas clouds where the whole cycle can start again (only this time with material which will have a slightly higher proportion of heavy elements than the original star).
Much of this lifecycle is already well understood, especially the main sequence stage of the star. In recent years the various ways in which stars of different masses finally die has become much better understood. Undoubtedly the area where the most work still remains to be done is in the area of star formation where the processes whereby stars form out of large clouds of gas and dust have not been very well worked out. Nonetheless over the last two decades much progress has been made with the advent of new instruments that enable much better viewing of these regions. These have lead to the identification of several key stages that a protostar has to pass through on its way to becoming a star on the main sequence.