Photographic film is made by coating a light sensitive layer atop a clear plastic support. Graininess is often seen in finished paper prints or when a slide (transparency) is projected on a screen etc. We select slow speed film (low ISO) when possible because slow films show less grain. Thus fast films show more grain. The degree of enlargement is a key factor. Prints 8X10 inch and large, often show grain. This is because 35mm is a miniature format. We enlarger these negatives about 8X or more to get to the 8X10 size. We mitigate grain by switching to a larger format camera and/or use low ISO film.

In the case of color print film, the grain you see is clumping of the dye clouds that make up the color image. Generally, 200 or 400 ISO color film fairs well when producing 8X10 prints. Graininess is increased by over-developing. Improper exposure will shows more grain. Grain is often more prevalent in areas of print that are mundane. We are talking uniform blue sky or any moderate density area that is uniform as to texture. This is because there is no busy patterns that tend to mask a grain pattern.

Again, your best bet, it grain annoys is to use 100 or 200 ISO color print film and expose as correctly as you can. Next roll, try bracketing your exposure. Make a series of exposures, some under some normal, some over exposed. This experiment will help you get a handle on grain.