Pond and water garden liners are available in a number of different materials all of which have the following in common:
Flexible, some materials will be more pliable than others and will lay flatter when installed;
Manufactured in sheet form with factory welded seams;
UV stabilized to help resist the effects of the sun;
Black in color for better reflecting values;
Fish-grade or potable grade (potable grade being suitable for use with drinking water applications);
Will last longer if buried or protected from direct sunlight. Expect 8-10 years or more from the least expensive lining materials, when protected, and 20-30 years or more from rubber lining materials. Water alone is a great UV inhibitor.
There are many differences between materials, such as: Price, how long the material lasts, weight and ease of handling, puncture resistance, method of patching, the ability to be field seamed and whether the material is reinforced or non-reinforced, etc.
Purchase your liner in a single welded flat sheet of material. It is better to have a liner too big than too small. It is easier to trim away the excess than to field seam and make the liner larger. Reliable seam integrity cannot be guaranteed when you attempt field seaming with glues and/or tapes. Field seaming presents some degree of difficulty for the average consumer and, consequently, should be avoided.
The primary advantages of flexible liners are:
The liners are easy to transport;
You are not limited in size or shape;
The materials will conform to your pond shape;
It is easy to make adjustments as you install;
There are a variety of materials available to suit just about every need;
The pond will have a natural look when surrounding landscape fills in bare areas.
Lining Materials has more detailed information on the different materials available