An informative essay educates your reader on a topic. They can have one of several functions: to define a term, compare and contrast something, analyze data, or provide a how-to. They do not, however, present an opinion or try to persuade your reader.
An argumentative essay tends to be a longer essay involving independent research, and aims to make an original argument about a topic. Its thesis statement makes a contentious claim that must be supported in an objective, evidence-based way.