Tariff
In contrast, in economic theory tariffs are viewed as a primary element in international trade with the function of the tariff being to influence the flow of trade by lowering or raising the price of targeted goods to create what amounts to an artificial competitive advantage. When tariffs are viewed and used in this fashion, they are addressing the country's and the competitors' respective economic healths in terms of maximizing or minimizing revenue flow rather than in terms of the ability to generate and maintain a competitive advantage which is the source of the revenue. As a result, the impact of the tariffs on the economic health of the country are at best minimal but often are counter-productive.
Trade agreement
A trade agreement (also known as trade pact) is a wide-ranging taxes, tariff and trade treaty that often includes investment guarantees. When two or more countries agree on terms that helps them trade with each other. The most common trade agreements are of the preferential and free trade types are concluded in order to reduce (or eliminate) tariffs, quotas and other trade restrictions on items traded between the signatories. 
The logic of formal trade agreements is that they outline what is agreed upon and the punishments for deviation from the rules set in the agreement. Trade agreements therefore make misunderstandings less likely, and create confidence on both sides that cheating will be punished; this increases the likelihood of long-term cooperation. An international organization, such as the IMF, can further incentivize cooperation by monitoring compliance with agreements and reporting third countries of the violations. Monitoring by international agencies may be needed to detect non-tariff barriers, which are disguised attempts at creating trade barriers.