Photos that are taken outside take advantage of one of the best light sources around: the sun. Sunlight tends to be rich and warm, which makes everyone look good. That said, when taking pictures outside, you should follow some basic rules.
First, avoid shooting on bright, cloudless sunny days. Too much sunlight can wash your subjects out. Plus, strong light means strong shadows. If you're taking pictures of people, those shadows can wind up on their faces (and no one wants to see the kind of shadow Aunt Ida's mole casts). Cloudy days are actually great for taking pictures. The clouds defuse the light, softening it but still showcasing rich colors. Plus, the diffused light will cast fewer shadows. If you have to take pictures on a bright sunny day, try to avoid taking pictures in the middle of the day, when the sun will be at its brightest. If you're taking pictures of people outside, taking them in the middle of the day can result in people squinting into the camera.
If it's a bright sunny day and you're taking pictures, look for ways to diffuse the light yourself. Open shade trees are a good way to do it. Look for a large tree with a wide spread of branches that are fairly high off the ground. The leaves will catch most of the light but will also let just enough in so your subject isn't completely in the dark. Just watch out for any harsh shadows -- especially on your subject's face.