This is a really common question for new climbers. And the answer isn't always obvious.
Climbers tend to use chalk to keep their hands dry while climbing. The primary reason that one's hands get wet is due to sweat. But humidity and natural water on a route can also make a climber's hands wet. Chalk can be used to counter these issues.
When we talk about chalk, we're not talking about the type you saw in elementary school. That type of chalk has a calcium carbonate base. Calcium carbonate crumbles and comes apart when it's wet, so it's not that great for climbing. Climbing chalk has a magnesium carbonate base, which absorbs water (or sweat).
There are three primary options for climbing chalk: liquid chalk, loose chalk and chalk balls.
Liquid chalk has really found it's niche as it is the primary chalk now allowed in rock gyms, due to the COVID-19 pandemic. Liquid chalk has a calcium carbonate base and is mixed with alcohol. When you put it on, the alcohol evaporates (and kills coronavirus!), leaving a thin layer of chalk on your hands.
The big upside to liquid chalk is that it tends to last awhile on your hands. The downside is that you can't really put it in a chalk bag, so it's hard to "chalk-up" mid-pitch. Additionally, if you have even the tiniest cut or nick on your hand, it will hurt a lot to use, as the alcohol will sting...
Loose chalk is primarily used by boulderers and is commonly put into a big chalk bucket. It is easy to spill and often shrouds a rock gym in a veil of chalky mist. I don't really use loose chalk that much, except to refill my chalk balls.
Some chalk comes as a brick that needs to be broken up into loose chalk. However, this tends to be a cheaper and less effective option.
Chalk balls are fabric balls filled with chalk that can be placed in a chalk bag. They often come filled, and can easily be refilled with loose chalk. As chalk balls aren't that messy and tend to last for awhile, this is my personal "go to" chalk.
The question as to whether you need chalk really depends on the type of climbing that you intend to do.
Most alpine climbing isn't that hard. The vast majority of the alpine routes that are regularly climbed in the world, are 5.7 or easier. And even when the routes are harder, the cruxes tend to be short. Chalk isn't really required on these kinds of climbs. You can usually get away without it.
If you are doing a harder alpine climb, you'll have to consider where you're going to hang your chalk bag. The standard spot, at your tailbone, will most likely be covered by a pack. Often alpine climbers that need chalk will offset their bag from their pack, on one hip or another. This usually means it's easier to reach with one hand or another. Chalk balls are easier in this setting, because the ball can be pulled out and used by either hand.
In most other climbing settings, chalk is a good idea. However, in some areas there are Leave No Trace considerations. Hikers and birdwatchers don't like to see chalk smeared all over a cliff face. That said, it is possible to buy colored chalk for certain areas. Make sure that you're aware of the local ethics before using any kind of chalk.
A classic chalk bag with a belt.
Finally, you should be aware that there are really two ways that chalk is carried. Boulderers often use chalk buckets, so that they don't have to carry the chalk. However, most other climbers use chalk bags, because they can be clipped to a harness or worn on a belt. If you're doing anything longer than an eight move boulder problem a chalk bag tends to be a better option.
--Jason D. Martin