An adhesive is a substance, which is used to join two or more parts so as to form a single unit. The application of adhesive has the following advantages over the conventional methods of bolting, riveting and welding.

1. A wide variety of combinations in joining are possible.
2. It can be used for bonding the surfaces of glass, metal, plastics and wood.
3. It creates a massive effect.
4. It is possible to prevent corrosion between different metals joined by adhesive.
5. It produces adequate strength.
6. Permeable joint can be made impermeable for water and gas by the application of adhesives.
7. The process of applying adhesive is easy, economical and speedy.

The disadvantages of adhesives are as follows:
1. It is not possible to adhesive for all substances. Depending upon the properties of substances to be joined, suitable adhesive has to be selected.
2. The adhesive substance does not become strong immediately after its application. It requires some time to attain the desired strength.
3. The adhesive substance does not generally remain stable at high temperature.

Following factors determine the quality of an adhesive:
1. Its degree or intensity of sticking,
2. Its durability,
3. Its resistance to heat,
4. The strength of bond developed after drying or setting,
5. The time required to develop the required bond, etc.

Glue is a general term, which is used to indicate an adhesive substance. Following are the various types of adhesives:

1. Albumin glues:
It is glue of better quality. It is not attacked by water. It is used foe making furniture.

2. Animal protein glues:
It is obtained by boiling waste pieces of skins, bones, etc. of animals with hot water. Animal glue develops strong and tough joints and it is easy to apply. But it is affected by damp and moist conditions. It is available in the form of cakes, flakes, granules, jelly, pearls and sheets. It is used in the manufacture of plywood, laminated timbers, etc.

3. Glues from natural resins:
It is prepared from natural resins. It is used for labeling, building paper, etc.

4. Glues from synthetic resins
These glues are based on synthetic resins. They may either be thermosetting glues or thermo plastic glues. Thermo setting glues become permanent, once they are set. The thermoplastic glues can be made plastic again by reheating.

All synthetic glues are fireproof, strong and water proof. The setting time for synthetic glues can be regulated by varying the type of kind and quality of hardener. They resist the attack by fungi and they possess non-staining qualities. The synthetic resins are of four main types:
(a). Melamine Resins: They require heat and pressure for setting. They are used in the manufacture of plywood.

(b). Phenolic Resins: They are available either in liquid, film or powder form. They require heat and pressure to form a permanent strong bond. They are used in the manufacture of resin-bonded plywood.

(c). Resorcinol Resins: They are in the form of dark viscous liquids. They are resistant to bacteria, fungi, heat and moisture. They become hard in short time at low temperatures.

(d). Urea Resins: They are available in the form of syrups and powders. They are extensively used in joinery work to form water resistant glue joints.

5. Nitrocellulose Glues:
It is prepared from pyroxilin, which is nitrated cellulose. It is derived by treating cellulose with nitric acid. It produces films, which strongly adhere to glass.

6. Rubber Glues:
It is prepare by dissolving rubber in benzene. It is used for joining rubber, plastics, glass etc.

7. Special Glues:
These are specially prepared to join metals. Cycleweld is a modified form of rubber and it is used to join aluminium sheets. Aradlite is another variety of special glue. It is used to join light metals.

8. Starch Glues:
It is prepared from vegetable starch. It has good strength in dry condition. But it is not moisture resistant. It is cheap and is used for inferior quality of plywood.

9. Vegetable Glues:
It is prepared from natural gums and starches. It is used for preparing paperboard articles, labeling etc.