If there is a cardinal rule in yardening, it is never use a chain saw while standing on a ladder. Leave that job to the professionals.
Chain saws will have chain guides from 10 inches to over 20 inches in length. Whether you buy an electric model or a gas model, most yardeners are best served with either a 14 inch saw or a 16 inch saw. Anything less is limiting in terms of what you can cut, and any larger is for people with major wood cutting requirements. If you are cutting down a tree, the rule of thumb is that you can cut down a tree with a diameter that is twice the length of the saw. With that said, I would never try to cut down a tree larger than one with a 12 inch diameter. Felling bigger trees can be very dangerous unless you know what you are doing. I leave those to professional arborists. With that caveat, the 14 inch or 16 inch saw will do most yard care jobs with ease.
When a chain is cutting wood, it is actually taking bites or chunks out of that wood at a very high speed. The effectiveness of the cutting process is directly related to the power that is driving that chain. If you overload an electric saw, you can burn out the motor. If you overload a gas saw, it will slow down and even stop cutting.
Anyone can learn how to use a string trimmer all by themselves. However, no one should use a chain saw without having an experienced cutter demonstrate proper technique and safety habits. If you can’t find a friend with lots of experience with a chain saw, check around to see if any retailer gives classes on chain saw operation. In any case, be sure to buy at least one book about chain saws and read it carefully before firing up that handy but dangerous tool.
Chainsaws come in lightweight, midweight and heavyweight (professional) models. Bigger is not always better with this equipment. A modern, lightweight chainsaw is an efficient tool that can be operated by a homeowner and handle a variety of yard chores. Use it for pruning large limbs from trees and shrubs, removing unwanted, small dying or diseased trees, cutting up broken and fallen branches, and shearing densely overgrown, woody hedges. A chainsaw is also great for handling brush and brambles and, some folks use them for cutting back large clumps of dried ornamental grasses in late winter, although this is not recommended by many manufacturers because of the increased chance of kickback.
Always handy for cutting firewood, a chainsaw can also be useful in construction projects such as fence and deck building where rough cuts will suffice.
For more information about pruning trees see the files on Pruning Shade and Flowering Trees and Choosing Pruning Tools
Always Use Ear and Eye Protection
Whether you are using a gasoline chainsaw or a electric chainsaw, the noise of those machines, when experienced for more than 30 minutes, can cause measurable harm to your hearing. Traditional passive ear protectors, like those used on a shooting gallery, do not provide sufficient protection. Go to the Yardener's Tool Shed and check out NoiseBusters