The process of photosynthesis can be compared to that of making sugar cookies. Just like you need certain ingredients to make cookies, you need certain ingredients to make Photosynthesis happen. The "ingredients" in Photosynthesis are light, Carbon dioxide and water. The sun (light) can be compared to the heat of the oven (since you need light for Photosynthesis to get going and you need the oven for the cookies to bake.) So once the cookies bake, you have the final product. In Photosynthesis, the end result is sugar(cookie) and oxygen. When you make cookies you can smell them. The smell eventually goes away. Oxygen is like the smell, which is just a by product of the entire process.
Plants and animals need sugar, specifically glucose. Plants and animals use Glucose to make ATP energy in a process called cellular respiration. ATP is critical for cells to carry out their cellular activities. Plants have light capturing molecules called pigments. One type of pigment plants use for Photosynthesis is called chlorophyll which is found in the chloroplasts of plant cells. Two major reactions that occur in the chloroplasts that together make up photosynthesis include- the light dependent reaction and the light independent reaction.
The light independent reaction is also called Calvin Cycle or dark reaction. Light dependent reaction happens in Thylakoids which are little compartments in the chloroplasts and they contain the pigment Chlorophyll. They appear to be nicely stacked and are called Granum which look like a stack of green pancakes. Light is converted into chemical energy by means of a complex process that involves multiple photosysytems. By chemical energy, we mean ATP and NADPH. Water (H20) which is needed for this process is split to get electrons, protons and oxygen. So, oxygen is also a product of this reaction.
Light Independent reaction also occurs in the chloroplasts. It needs items from the light dependent reactions, so they happen at the same time. It does not happen in the Thylakoids. Instead, it happens in the Stroma, the fluid outside of the Thylakoid. In this reaction, Carbon dioxide must be fixed with the additional help of a major enzyme. The inorganic carbon dioxide is changed to a more usable organic form. The ATP from the light dependent reaction helps supply energy and the NDPH from the light dependent reaction supplies 'reducing power' which means that it helps add high energy electrons to this process. Therefore, in a very complex series of pathways, the fixed Carbon Dioxide, the ATP and the NADPH are used to make a product that can ultimately be converted into glucose.