You will find there is a great deal of variation in the timber used on garden buildings. On the very cheap sheds you are likely to find that the planks of wood for the walls are just sawn timber (i.e., will have a rough finish) and each of these boards will overlap the one above. Some boards are cut in a feather edge finish (i.e. If you look end on to the plank it will appear triangular) and these overlap each other. In my view these are better than the square planks of wood as the feather edge makes for a smoother finish.
Both of these are OK if you are on a small budget but it's vital that the timber is not wafer thin. You need to ensure that the boards are at least 12mm thick so the wood can cope with changes in weather conditions. Any less than that and there is a good chance the boards will split during any long hot spells of weather. You will find that some of the cheap sheds are made from boards only 6 mm thick which is crazy. These boards will not last more than a few years at best.
As important is how much the boards overlap each other. Ensure that there is at least 22mm (¾ in) on each board. Any less than this you will find that when the boards shrink during any hot spell gaps will appear. Always ask the question BEFORE you buy and don't be fobbed off with any fluffy answers. I do know you need at least that amount as I made sheds for over 20 years.
The next popular option is shiplap cladding. Traditionally this cladding was flat with a lip on the top and channel on the bottom so they fit together. This ensured a smooth surface on the inside and a profile on the outside. This is how shiplap was in the early 1980's.
I asked a timber company at that time, when I was making garden sheds, whether they could put a tongue and groove (T&G) on the boards as I felt this would make for a better finish. They did this and from then on we only used the T&G on our garden sheds. Amazingly after that many more timber suppliers and garden shed manufacturers started using T&G shiplap as well.
So I think I can claim to be responsible for T&G shiplap. The beauty of this is that the boards slot in together so when a shed side is completed it effectively becomes one complete side. Nearly impossible to prise boards off to gain entrance unlike overlap shiplap, feather edge or square boarding. Some sheds are made from square T&G boarding which can look a little more utilitarian.
Another popular option is loglap cladding. Essentially this is the same as shiplap except that on the outside of each plank of wood there is a rounded effect which gives an log cabin effect, sort of. On the inside walls it is smooth. Again try to ensure you insist of T&G boards as this helps to strengthen the walls against intruders. If you are offered overlap loglap be a little wary.
On some of the thicker loglap, such as the Platinum Workshops and Garden Studios, there are grooves cut on the inside of the boards and this helps to stabilise the boards from 'cupping'. This is when the boards try to go back to their natural shape.
The key to choosing is quite often cosmetic but economics can come into your sums. However, if you are on a budget think carefully about the cheap sheds being offered. These sheds are cheap because the material is not as good as the better sheds. Nothing overly wrong with that provided they last a number of years before they need renewing. If you are only needing a shed for a few years then they may do the job. But when you come to move and you have a garden shed in your garden in a right state this may put of any potential purchasers. Just a thought before lashing out on a new shed.
Where ever possible try to view the garden shed before buying and also take a look at any INDEPENDENT customer feedback sites. This usually gives you a good idea of the quality of the product as well as how good their customer service is. You will often find with the cheap shed companies they don't want to know after they have got your money.
Another important thing to consider is what type of garden sheds and where your new garden shed should go. Take a look at this page for information explaining the what to look out for.