Saline solution is not inherently sterile, but most commercial preparations are designed to be sterile. Saline solutions, as defined by About.com, are any water-based mixtures that contain salt, so even non-sterile mixes that contain salt are, technically, saline solutions.
The saline solutions used by hospitals are a specific type of 0.9% sodium chloride prepared for injection, as Forbes notes. About.com delineates a simple process by which sterile saline can be made at home; however, commercially prepared medical saline can take 3 weeks to make per batch, as described by Capt. Valerie Jensen in her interview with National Public Radio. Saline meant for certain medical applications requires extensive aseptic processing, and even correctly made sterile saline quickly becomes contaminated when exposed to air, About.com emphasizes.