Mason sand is finer than fill sand or utility sand, thus it fills the spaces between pavers more completely and with fewer crevices. The problem with using masonry sand is that weeds and grass can grow through it. Mason sand comes in a mixture of light brown/tan colors or in white. It is available from suppliers in bags or in bulk.
Polymeric sand has plastic polymers mixed into it that expand and set when wet. Polymeric sand doesn't allow weeds or grass to grow in the cracks. It is only available in bags and is the most expensive alternative for locking pavers.
If your paver hardscape is not ever going to be removed, you can use a half-and-half mixture of cement and mason sand to fill the cracks. When wet, the cement sets and forms a permanent seal. This method has several drawbacks. First, you have to be sure none of the mixture is left on the pavers before you wet it down or it will set on the pavers. Secondly, cracks may develop and allow weeds through. The final drawback is that having no room for the pavers to move can cause upheaval and an uneven surface after a floods or freezing weather. The cement mixture adheres to the pavers, making them impossible to reset once disturbed without removing the cement, which in turn may damage the pavers.
All the fillers are applied in the same way. The filler is spread over the surface and swept into the cracks. The surface is then wet down with a light spray of water, which washes the filler into the cracks and sets the filler. With plain mason sand, you may have to reapply once the first application has settled.
If weeds break through the gaps, you will have to use weed killer to get rid of them. The gaps are usually too small to allow for pulling or digging of the weeds. Sand often washes out from between the pavers, necessitating re-application.