A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of athylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as grana (singular: granum). Grana are connected byintergrana or stroma thylakoids, which join granum stacks together as a single functional compartment.
Thylakoids are the membrane bound specialized structures in chloroplasts where the light-dependent reactions of photosynthesis occur. This is important as the light-dependent reactions are responsible for utilizing solar energy to to oxidize water and create energy storing ATP and NADPH. In order to maximize the efficiency of these reactions and photosynthesize as much as possible, thylakoids usually occur in large stacks called grana that reside inside the thick fluid stroma of the chloroplasts.
There are stacks of thylakoids in the chloroplast, in the membrane of the the thylakoids light-dependent reactions occur, and in the thylakoid space (or cytoplasm of the thylakoid) the Calvin cycle (or light-independent reaction) occurs. If you haven't gotten this far into it, photosynthesis occurs in the chloroplast.
The chloroplast is an organelle found in plant and algae cells. It is found in the part of a plant that contains chlorophyll and where photosynthesis takes place. Chloroplasts have two membrane layers (an outer and inner membrane). The inner membrane layer contains stacks of flattened sacs (thylakoids) called grana. The chlorophyll is found in the thylakoids.
Grana are stacks of thylakoids that lie in the stroma (fluid filled space in the chloroplasts). It is where the light reactions of photosynthesis takes place (light and water makes oxygen). After these light reactions in the grana/thylakoids, there are the dark reactions which take outside of the grana, in the stroma. These dark reactions use carbon dioxide to make carbon sugar molecules.