A knife is primarily a tool, and as every good tool, it needs good maintenance.
The first maintenance is to keep it clean, always wipe the blade after use.
The mechanism, friction, slipjoint or locking, needs to be cleaned and oiled regularly. A simple food grade mineral oil is perfect and safe if you use your knife to eat.
Wood and horn handle need specific care. Wood can be oiled using a regular furniture polish or oil. A horn handle might dry and crack over time, some hand moisturizer or oil every once in a while will help.
Do not let a knife with a wood, horn or bone handle in water and do not put it in a dishwasher.
When carrying your knife in your pocket, avoid mixing it with keys or coins, as it will scratch the handle or bolsters.
Although scratches are part of the fun of seeing the knife ageing, they can easily be polished with a polishing compound or wipe, attention however with bone or wood handles that can be stained by that compound.
Of course, we like to have several knives and have to store them. Avoid humidity or direct sunlight and extreme temperature that could cause damage to natural handles.
Finally, a knife needs to stay sharp, it is what it is made for and much less dangerous than a dull blade that slips, rip… and still cut enough to hurt yourself. A little bit of honing regularly will help keeping the blade sharp without too much effort. Never sharpen a blade on a power driven grinding wheel as it might overheat and alter the temper of the blade.
The carbon steel case.
Unlike stainless, carbon steel blades (like XC75 and 1095) will rust and require special care.
Keep away from moisture and wipe off fingerprints, those two will be your worst enemies.
Like other knives, a drop of mineral oil will greatly help, especially in the mechanism. You can also apply a slight coat of oil on the blade. Avoid olive oil; its acidity will in fact create oxidation.
Finally, a carbon steel blade will change color over time, even just by being in contact with ambient air, and develop a patina, it will also change color as you use it, especially to cut fruits, and, of course, especially citrus. This is part of the charm of a carbon steel but can easily be polished off.