Yerba Mate (Ilex paraguariensis) is a species of the holly (family Aquifoliaceae) native to subtropical South America in Argentina, Uruguay, Brazil, Chile, and Paraguay. It was first scientifically classified in the late 1800's by a swiss botanist named Moses Bertoni. Yerba mate is a shrub or small tree that can grow up to 15 meters tall. It has evergreen leaves, and has small, green / white flowers. It also has a fruit that measures 4 to 6 mm in diameter. 
Yerba Mate contains caffeine and theobromine, both of which are stimulants. Studies have identified and quantified the amount of caffeine and theobromine in the leaf waxes of Yerba Mate. The total leaf wax content was 0.5% of the dry leaf weight, and the caffeine and theobromine contents of the leaf wax varied from 0.16 to 127.6 mcg/mg and from 0 to 9.5 mcg/mg of wax respectively. 
The physiological effects of Yerba Mate is rather similar to other caffeinated drinks such as coffee, tea, and guarana beverages. Yerba Mate tea generally increase the focus and alertness of the user, which is common to stimulants. On the other hand, because it contains stimulants, users may also feel various negative side effects such as anxiety and jitteriness. Because of its stimulant effects, Yerba Mate is a common ingredient in fat burners and diet pills. In various regions across the world, Yerba Mate tea is a popular drink for its many health benefits; however, there are also some potential danger and downside to consuming too much Yerba Mate. We will discuss and explore all these issues in this review.
Yerba Mate Tea Information
Yerba Mate tea is an infusion made from the leaves of the tree Ilex paraguariensis. The mate tea is a widely consumed nonalcoholic beverage in South America, and is also gaining increased popularity around the world as well.
Yerba Mate has long been used as a medicine, and also to make beverage. It stimulates the central nervous system, has benefits for the cardiovascular system, and there are indications that it can also help with weight loss and weight management. This is a large reason why you find Yerba Mate in many different fat burner and weight loss supplements. It has also been reported that Yerba Mate tea is associated with both the prevention and the cause of some types of cancers. 
There are various clinical studies that have found Yerba Mate tea to have various health benefits including having antioxidant properties. Antioxidant activity of Yerba Mate tea was observed in vitro and in animal models. In a Brazilian study conducted at the University of Sao Paulo, the antioxidant effects of Yerba Mate tea on health young women was studied. The study results showed that after the supplementation period with Yerba mate tea, lipid peroxidation was acutely lowered, an effect that was maintained after prolonged administration. This study also found that total antioxidant status and the level of antioxidant enzyme gene expression were also demonstrated after prolonged consumption. They concluded that regular consumption of mate tea may increase antioxidant defense of the body by multiple mechanisms. 
Health Benefits of Yerba Mate
Aside from the antioxidant effects of Yerba Mate tea as mentioned above, other health benefits of Yerba Mate include:
Yerba mate is anti-inflammatory effects
It helps improve cognition and memory
Yerba mate also shows some promise in treating obesity, and helping with weight loss
Short term cigarette smoke exposure has been associated with acute lung inflammation and oxidative damage. In a study done on Yerba mate tea and acute lung inflammation in mice exposed to cigarette smoke, Yerba Mate tea (150mg/kg) was administered to the mice, and were then exposed to cigarette smoke for 5 days at a rate of 6 cigarettes per day. The results of this study found that the Yerba Mate tea reduced lipid peroxidation (a process whereby free radicals "steal" electrons from the lipids in cell membranes, resulting in cell damage). Their data shows that there is a potential antioxidant role for yerba mate tea on acute lung inflammation, and that it has anti-inflammatory potential; however, they also stated that further studies are needed to investigate these effects. 
Yerba Mate (Ilex paraguariensis) is used to prepare a tea like beverage, and it has a reputation of helping to improve cognitive function, a response that has been attributed to the constituents of the leaves, such as caffeine. Several studies have been done on lab animals to test this, and the results have demonstrated cognitive benefits of Yerba mate. One Brazilian study tested the effects of a yerba mate extract on the short and long term learning and memory of rats. Oral administration of the Yerba mate extract improved short term social memory in a specific manner. Their findings partly substantiated the traditional use of Yerba mate tea for improvement of cognition. 
1. Wikipedia (Yerba Mate) - http://en.wikipedia.org/wiki/Ilex_paraguariensis
2. Phytochemistry. 2000 Dec;55(7):853-7.
Caffeine and theobromine in epicuticular wax of Ilex paraguariensis A. St.-Hil.
Athayde ML, Coelho GC, Schenkel EP.
3. J Food Sci. 2007 Nov;72(9):R138-51.
Yerba Mate Tea (Ilex paraguariensis): a comprehensive review on chemistry, health implications, and technological considerations.
Heck CI, de Mejia EG.
4. J Agric Food Chem. 2009 Feb 16.
Effects of Mate Tea (Ilex paraguariensis) Ingestion on mRNA Expression of Antioxidant Enzymes, Lipid Peroxidation, and Total Antioxidant Status in Healthy Young Women.
Matsumoto RL, Bastos DH, Mendonça S, Nunes VS, Bartchewsky W, Ribeiro ML, de Oliveira Carvalho P.
5. Nutrition. 2008 Apr;24(4):375-81. Epub 2008 Feb 20.
Mate tea reduced acute lung inflammation in mice exposed to cigarette smoke.
Lanzetti M, Bezerra FS, Romana-Souza B, Brando-Lima AC, Koatz VL, Porto LC, Valenca SS.
6. J Ethnopharmacol. 2008 Dec 8;120(3):465-73. Epub 2008 Sep 27.
Effects of acute administration of the hydroalcoholic extract of mate tea leaves (Ilex paraguariensis) in animal models of learning and memory.