Bikes parts are greased and lubed for protection and reduced friction. The grease and oil attracts dirt, which can wear down parts, muck up shifting, and looks ugly.
There are many products for degreasing and cleaning, and it's important to pay attention and handle chemicals safely and avoid damaging parts with the wrong one.
WD-40 is a popular spray degreaser, which liquefies grease so it can be wiped away. However, it leaves a residue that can attract more dirt, and makes a poor lubricant for bikes. There is a new WD-40 bike specific degreaser with a different formula that is a better option.
I avoid sprays because they get everywhere. I might spray degreaser on a rag and wipe parts down, and will pull them off for a deep clean when necessary.
Regular WD-40 is generally not recommended for bikes, and is mentioned first as a PSA.
Mineral spirits are a good choice for soaking chains and cassettes, it will dissolve grease and not harm parts. It's pretty nasty stuff, so save it for reuse in a glass container instead of pouring down the drain. After it sits, metal and dirt sink to the bottom and you can pour the clean spirits into another container. Rinse with alcohol to get remaining residue off before relubing.
Stores are selling a different substance as mineral spirits. The "green" version has a milky white appearance and is more environmentally friendly, but may not work as well..
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Simple Green can cause hydrogen embrittlement on steel parts if soaked for a long time, and not diluted. They make a bike specific cleaners, and non corrosive Simple Green Purple, but you are better off with mineral spirits if you have the choice.
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Denatured alcohol, or methylated spirits are a must for rinsing residue off parts after degreasing. Since they will strip chains of any protective lubricants, make sure to lubricate or wax soon after they dry.