Understanding eyeliner requires understanding eyeliner styles. You must understand the kinds of eyeliner available to you and the eyeliner look you seek in order to make your eyes pop in the manner you would like. If you don't, you're going to be trying to use liquid eyeliner for a natural look and wondering why on earth the whole look just seems wrong.
While different styles of eyeliner are displayed side by side in beauty department store aisles, that's really where most eyeliner similarities end. The looks that are created using different kinds of eyeliner vary so widely that each particular liner should be considered its own individual cosmetic tool. While one might be used for subtlety, another will only be used for instant glamour; it is altogether likely that a fashionista will have specific eyeliners that are only worn with specific colors of clothing. It's that specialized.
If you're going to give your eyes the dramatic cat eye look, you want liquid liner. In fact, you probably want onyx black liquid liner. This -60s-reminiscent look is all about clean, crisp lines along the upper and lower lid of your eyes, with a sassy little upturned flair at the outer corner. The best way to achieve this smudge-free look is with a liquid liner that will go on smoothly. A soft eye pencil will create a sloppy take on a look that is based upon the idea of polish.
If you're looking for a heavy smoky eye, you actually don't even need to pick up a liner at all. Your eyeshadow will provide the emphasis here. Eyeshadow serves as a great liner when applied correctly - if you lightly wet your eyeshadow brush, dip it into shadow and apply in a thin line, it will provide a soft line of color that you can smudge accordingly. This also is a great introduction to people new to working with the concept of liner.
Natural and makeup-free are two different concepts. A natural eye will let the beauty of your eyes' color and shape stand out, but relies on cosmetics to showcase them to their highest potential. If you want to create a natural eye look, stick with a neutral-colored eye pencil. Pencil will let you lightly smudge the liner so that it becomes less obvious.
If you want to line your eyes with lime green or an electric blue, you can utilize any liner style you'd like. Get creative and have some fun to see whether a liquid liner or pencil best suits your needs.