Water-levels and bunyips are inexpensive easy to make tools that outperform some high tech costly laser levels in many situations. There are at least three basic DIY designs for different applications. All involve clear tubing and of course water.
In its simplest form all is required is a tube. You partially fill it with water. Set the water line (aka meniscus) at the height you want and the water line on the other end of the tube no matter how far away is guaranteed by gravity to be at the same height. Appropriately sized corks for each end make transport around the site less likely to become damp. Here's an explanation of this simple form of water level by Kelly Smith on DIY Life.
A reservoir or bucket and tube water level will be more accurate on a landscaping project where the goal is for a number of widely spaced objects to be absolutely level, or whenever you want multiple marks to be at exactly the same height. The bucket or reservoir stays in place as you move the one tube so that the water line at the end of the tube will always be at the level of water in the stationary bucket. Dale Williams on factsfacts.com has an excellent explanation of how to make a bucket style water level along with a second post explaining a a bit more on how to use it. Buildeazy.com has an illustration that may make it clearer how this type of water level is used for fence and deck construction on sloping property.
A bunyip or "Egyptian water-level" is the tool to use when you want to measure differences in elevation. This is used for creating water catchment earthworks among other things, when you want to make sure that the bottom of your earthwork is a certain distance below the top or that your french drain has a proper slope.
A bunyip water level requires a measuring stick, rod or post at each end to which the tube is attached. (Yardsticks can be used but typically the sticks used are longer and marked by hand.) When the bottom of each post is placed on the ground, the difference in elevation will be shown by difference in the from the meniscus on each to the top of the post or stick.
For example if the meniscus at one end is four inches from the top of the stick and the meniscus on the other end is ten inches from the top you know that the ground at the second site is six inches higher.
Here's a video by zhiniibones of Tuscon, Arizona that's the most complete explanation and demonstration on how to make a bunyip that I've run across.
If making your own seems too difficult or if you have more money than time you can also purchase a wide variety of water levels at watrlevel.com. They make water levels specifically designed for building decks, foundations, making patios, mobile home leveling, installing siding laying brick, concrete forms, laying lawns, installing cabinets, grading home inspection, t-bar installation, pond construction and installing rain gutters.