Photosynthesis is the process with which plants make their food. It is derived from the Latin 'photo' (light) and 'synthesis' (composition, putting together). In order for plants to make their food, they need the certain molecules and compounds to be able to produce cellulose, a kind of sugar. The plant needs water (H₂O) and Carbon Dioxide (CO₂). It adds light and chlorophyll to these molecules and the result is Cellulose (C6H12O6) and Oxygen (O₂). The plants don't need the oxygen and so they let it out. Plants are useful for us because they take in our waste, (CO₂) and their waste is a vital part of life for us (O₂). Photosynthesis can be written as:
6H₂O + 6CO₂ ----light+chlorophyll----> C6H12O6+ 6O₂
Think of it as a recipe. The plant needs all the ingredients to make a cake. The plant needs water and carbon dioxide. Then the plant needs chlorophyll and light, which could be seen as the oven. When the cake is cooked, you get your food, but you are alone and you don't need all that cake, so you cut off what you know you won't eat and you throw that piece out. Someone passes by and takes that piece of cake., i.e. another organism takes the energy not needed by the plant. The formula seen above is merely the recipe written on paper. It means that six molecules of water and six molecules of carbon dioxide plus sunlight and chlorophyll makes one molecule of sugar and six molecules of oxygen.
Plants need to find their ingredients before starting to bake the cake, and they are very well adapted to find and procure their necessary ingredients. The roots take care of sucking up the water in the soil. This water is transported to the leaves through the stem. Photosynthesis occurs in the leaves. The chlorophyll is stored in little pockets within the leaves known as chloroplasts. Chlorophyll is a green pigment (colouring), which is why leaves are green in general. The carbon dioxide needed is taken in from the stoma, which can be characterised as little pores seen on the underside of leaves. The unwanted oxygen is also excreted from the stoma.
Plants are a crucial part of life, they are the only organisms capable of taking energy from the sun to transform it into heat energy, which other organisms steal from plants. Plants also uphold the balance of gases in the air by breathing in carbon dioxide and breathing out oxygen.
References: Animated Image From Millan.Net, http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookps.html, http://en.wikipedia.org/wiki/File:Leaf_anatomy.svg, http://www.folens.ie/catalog/book_detail.php?bookid=91,