Depending on the context, resolution has many definitions. As it relates to digital photographs, resolution is the amount of pixels within the image. Terms such as high-resolution or low-resolution photo may be used. "High" is relative to the final output or print size. Hence, what may be considered great resolution for a business card may have far too little detail for a large poster.
It is best to capture images with the highest resolution (most megapixels) available and then scale them down to the proper size. Many consumer cameras advertised with multi-megapixels come from the factory with a setting much lower to store more pictures. For this reason more aggressive compression may also be the default. These factory settings may result in images that look good on a computer screen and when printed at reasonable sizes. However, for large posters and high quality printing, minimal compression and increased resolution is required to avoid noticeable artifacts.