Thylakoids (sometimes spelled thylakoïds), are small interconnected sacks which contain the membranes that the light reactions of photosynthesis take place on. The word thylakoid comes from the Greek word thylakos which means "sack".
Suspended within the chloroplast stroma is the thylakoid system, a highly dynamic collection of membranous sacks called thylakoids where chlorophyll is found and the light reactions of photosynthesis happen.
In most vascular plant chloroplasts, the thylakoids are arranged in stacks called grana, though in certain  plant chloroplasts and some algal chloroplasts, the thylakoids are free floating.