Whoever invented guitars was not thinking about how to carry them. No matter what you try, from tucking the body under your arm to slinging it over your shoulder by the neck, its awkward and unwieldy. Try carrying it through a revolving door—you get the picture. But a gig bag opens up options, including carrying your instrument by a handle or wearing it backpack-style, which makes transport manageable. Look for these things when choosing a gig bag for your guitar.
Gig bags do come in various sizes, to fit acoustic, electric, and bass guitars, and some are designed to fit cutaways or the uneven body shapes of certain electric guitars. Be sure it will fit your guitar. In addition, look for features like an easily gripped handle and padded, adjustable backpack straps for carrying comfort.
Some gig bags are cheaply made and should be avoided. Choose a bag made with ballistic nylon or other durable, tear-resistant material, high quality and well-fitted zippers, and reinforced stress points at strap and handle attachments.
A good gig bag is lined with padding and soft fabric for protecting your guitar. Some feature neck support and Velcro straps to secure your guitar inside the bag. Accessory pockets are a nice touch.