Batting comes prepackaged in standard sizes for crib, twin, full, queen and king size quilts, which is handy because it saves you a step. Or, you can buy batting off the bolt in your own custom size. This requires you to get your custom size cut at the store, or cut it yourself at home, but if you quilt a lot, off-the-bolt can be a more economical and convenient choice.
The most common quilt batting is made of cotton or polyester, but wool, silk, bamboo or a poly-cotton blend are all fair game. Batting also comes in blends that are organic (safe and recommended for baby quilts) or made from recycled fibers. Here's the good thing: there's no right or wrong here — fiber's mostly about personal choice.
Loft is basically a fancy way of saying thickness. If you go for high-loft batting, the lines of your quilting will be more visible and the quilt will be puffier overall. Low-loft batting gives a flatter finish, which is great if you want to show off the piecing more than the actual quilting lines.
Quilters are definitely a loyal bunch, and most have a favorite batting brand or two. If you're a newbie, go ahead and experiment, but also feel free to lean on more experienced quilters for recommendations. Warm & Natural batting is known for creating a soft, crinkled finish after washing that some quilters prefer, and Hobbs brand is a reliable choice.
Professional longarm quilting machines can handle pretty much any batting you can buy. But if you're using a domestic sewing machine, you might have an easier time with lower loft — especially if your project is on the large side. The bulk of large batting cuts, combined with thicker loft, can make it a tight squeeze to fit your basted quilt through the neck of your machine, and that's just asking for problems.
Once you've got the basics of size, fiber content, and brand, it's time to get into the nitty gritty. Here are some other terms you'll see, and what they mean.