Two Guitar Gig Bag Types
There are essentially two types of gig bag: a standard bag and a soft case. A standard bag is basically just that, a bag. A soft case is also basically what the name implies a soft case. The central difference is padding and levels of protection.
Standard Gig Bags
A standard bag will typically be just a bag that the guitar slips into, it may have a pocket or convenient strap but it is essentially a bag that will provide limited protection from bangs and knocks, temperature or humidity. This is the kind of bag that you will often see included in beginners starter packs or on the much lower end of the gig bag range. They have their place but are essentially to protect your guitar from dust, light scratches and light rain while you transport it from one place to another. If all you need to do is move the guitar from one place to another over a short distance I would say these are fine. All you need to look at with one of these is to ensure the strap is sturdy enough not to break while carrying it, will the zip hold, does it fit your guitar? However if you are carrying your guitar beyond round the corner by hand or have a more expensive guitar you want to protect I would recommend spending the extra and getting a guitar bag that fits into the soft case category.
Soft Case Gig Bags
A soft case gig bag will do everything a standard bag will do but to a much higher level plus provide padding to your bag that will help protect the guitar from knocks and bangs as well as insulate against temperature and humidity. Even if just going around the corner if I'm honest, I use a soft case gig bag. Sat next to you in a car in a bag your guitar can still fall and get knocked. In a car boot or back of a van it can slide into things or have other instruments fall on it. Even walking around a street or inside a building on the way to a gig your guitar can take a knock on a corner, door handle or door frame. Every time it does you will be happy your guitar has some protective padding around it (or wish it did if it doesn’t). Although a soft case bag still won't provide the same protection as a hard case it can still do a good job while on the move and is often noticeably lighter.
So how does this padding work if it's still made of material as a standard bag? Well some come with up to an inch (25mm) of padding, often a combination of soft and thick foam. Some also come with extra cushioning on the bottom and around the neck or headstock, much like a hard case, to provide that extra protection on the areas most commonly damaged on a drop (the bottom and neck). Often manufacturers who make soft case gig bags will pay more attention to the strength of the straps as well as water proofing it and making it overall stronger and so safer for your guitar.