from here http://www.crisp.nus.edu.sg/~research/tutorial/image.htm a good definition for pixel size and spatial resolution that applies to satellite imagery.

"Spatial resolution refers to the size of the smallest object that can be resolved on the ground. In a digital image, the resolution is limited by the pixel size, i.e. the smallest resolvable object cannot be smaller than the pixel size. The intrinsic resolution of an imaging system is determined primarily by the instantaneous field of view (IFOV) of the sensor, which is a measure of the ground area viewed by a single detector element in a given instant in time. However this intrinsic resolution can often be degraded by other factors which introduce blurring of the image, such as improper focusing, atmospheric scattering and target motion. The pixel size is determined by the sampling distance."

At the same link, you can find also some nice examples for clarifying the differences between 'Spatial Resolution and Pixel Size'

Hi Zohreh,

I would say Sheikh has given a good explanation ...

The grid size determines the scale of the map. The larger the grid, the more information is averaged.

from here http://www.crisp.nus.edu.sg/~research/tutorial/image.htm a good definition for pixel size and spatial resolution that applies to satellite imagery.

"Spatial resolution refers to the size of the smallest object that can be resolved on the ground. In a digital image, the resolution is limited by the pixel size, i.e. the smallest resolvable object cannot be smaller than the pixel size. The intrinsic resolution of an imaging system is determined primarily by the instantaneous field of view (IFOV) of the sensor, which is a measure of the ground area viewed by a single detector element in a given instant in time. However this intrinsic resolution can often be degraded by other factors which introduce blurring of the image, such as improper focusing, atmospheric scattering and target motion. The pixel size is determined by the sampling distance."

At the same link, you can find also some nice examples for clarifying the differences between 'Spatial Resolution and Pixel Size'

Interesting question. I use them interchangeably meaning they are the same thing. Depending who you talk to a remote sensing or a spatial analysts.   

Pixel size on the ground can be viewed as a physical property of your image. Also pixel size can be viewed as a physical property of your imaging sensor and it is a fixed quantity. Indeed, pixel size of the sensor is one of the factors that controls the pixel size on the ground. For example, IKONOS 1 m spatial resolution or pixel size refers to spatial coverage of one pixel on the ground, which is also called Ground Sampling Distance (GSD). On the other hand, the grid resolution or size is a design parameter that will be selected according to the purpose of the survey. For example, your soil survey grid size can be set at 500 m x 500 m or 250 m x 250 m; and once again it depends on the purpose of your survey. A complex relationship could be developed to relate your pixel size to the grid size, but this relationship has to consider several factors such as geology of the area, topography, rainfall, etc..