Most lawn and garden tools have at least some parts that are prone to rust. The alloys used typically contain iron, which combines readily with the oxygen in air and water and produces the familiar reddish paste, iron oxide. Iron oxide is rust. Actually any metal which oxidizes is said to be rusted, but aluminum oxide, zinc oxide and others are not usually a large problem with tools.
The old saying, an ounce of prevention is worth a pound of cure, is true in this case. Keeping tools clean and dry is the best and easiest first step. Moisture accelerates rusting. To prevent that, store smaller tools in a wooden box or add a packet of desiccant to the tool box. Hanging them will help keep them away from surface water, but they're still subject to airborne moisture. Sometimes, that's enough, however, depending on local climate.
For better prevention, oiling with a fine machine oil or coating with a paste wax will keep them dry and rust free. Of course, the coating will have to be re-applied after use and it's important to know what plants might be affected by the oil or wax. Most will do just fine.
If a tool becomes rusted, acting early will help prevent the rust from pitting or weakening its parts. Once rust starts, it tends to act like a virus and spread to the interior. The further it progresses, the harder it is to remove and the more damage it does to the tool.
In many cases, a bit of mineral oil wiped on the rust patch, followed by a light scrubbing with steel wool, will do the trick. Brush from the center to the tip, away from any sharpened edges and away from your body. That helps keep the edge sharp and minimizes accidents.
In more extreme cases, it may be necessary to use a commercial rust remover. Before getting to that stage try this old clock repairman's trick. Prepare a broth of two cups of boiling water and several black pekoe tea bags. More water and tea bags for larger parts. Let them dissolve for a minute until the liquid is very dark. Then immerse the parts for up to 8 hours, depending on the need.
The tannic acid in the tea will often act to remove or at least coat rusted areas, but more gently than the stronger chemicals used in commercial rust remover. Wipe the gray powder gently with steel wool as described above.
When it does become necessary to use the stronger commercial rust removers, leave the part in contact no longer than necessary. Most, such as those sold as 'naval jelly', contain phosporic acid. It does a fine job of removing rust and leaving a healthy coat of iron phosphate, which hinders rust formation.
But it does etch metal. That leaves a clean, smooth, shiny surface which is desirable. But carried too far it can remove excessive amounts of metal. That can create a gap, leaving shears and similar tools useless.
Remove the rust remover per instructions, then remove any remaining residual rust (if any) with steel wool. If necessary resharpen pitted blades. Then coat with oil or wax to prevent any future oxidation.