Photosynthesis takes place in chloroplast. Chloroplasts contain a pigment, chlorophyll, which allows the plant to harness solar energy and use it to manufacture energy, food and building materials for the plant. Chlorophyll is also the pigment that imparts the green color to plants. It absorbs light within the blue and red wavelength region and thus reflects green light.
Since they have to capture sunlight, chlorophyll, and hence chloroplasts are situated towards the periphery of a leaf if you look at a leaf's cross sectional area. This area is the palisade layer.
Structure wise, chloroplasts are flat disc like structures that are about 2 to 10 micrometer in diameter and about a micrometer thick (may vary from plant to plant).