An allergen is any substance that causes an allergic reaction, most often when eaten or inhaled.
No comprehensive list of allergens is possible, because sensitivities vary from one person to another and it is possible to be allergic to literally anything.
Dust, pollen and pet dander or pet hair are all common allergens, but it is possible to be allergic to anything from chlorine to perfume. Food allergies are not as common as food sensitivity, but some foods such as peanuts (really a legume), nuts, seafood and shellfish are the cause of serious allergies in many people. A few people have even been recorded to be allergic to certain chemicals found in almost all water. Poison ivy (and other plants, like poison sumac or poison oak) is a plant that will cause an allergic reaction for anyone, given enough repeated contact -- like any allergy, the human body must learn to fight the allergen, some bodies learn slower and will appear to be "immune" to poison ivy.
An allergic reaction can be caused by any form of direct contact with the allergen - eating or drinking a food you are sensitive to (ingestion), breathing in pollen, perfume or pet dander (inhalation), or brushing your body against an allergy-causing plant (direct contact, generally resulting in hives). Other common causes of serious allergy are bee stings, penicillin, and latex. An extremely serious form of an allergic reaction, which can kill in mere minutes, is called anaphylaxis.