ISO is a way of measuring the sensitivity of both digital camera sensors and film. By increasing the Iso it is possible to reduce the shutter speed or the aperture size which in-turn will increase the depth of field. The downside to increasing the iso level is the noise in the image is increased. If you are just taking holiday snaps I would advise you to let the camera choose the iso level. That way the camera can avoid camera shake by decreasing the shutter speed and increasing the iso level in low light situations. However if you are taking stock photography anything above iso 100 runs the risk of being rejected for excess noise.
The picture on the left was taken in Teeside of an outdoor market in bright sunshine. It proved to be so popular I decided to get some similar images from my local indoor market (right). However despite many attempts the higher iso the camera used resulted in all the images being rejected for too much noise. Before leaving for the Egypt trip I set both cameras (E-510 & E-330) to only shoot at the lowest iso setting (Iso 100). This approach resulted in very few images being rejected for noise but more being rejected due to shallow depth of field. One way around this problem is to use a professional stock camera which can shoot at iso 400 with almost no visible noise. The downside to this plan is such cameras cost as much as a small car.