A trade association, also known as an industry trade group, is an organization founded and funded by businesses that operate in a specific industry. Trade associations sit at the heart of their industry and are often the best sources of data and analysis on their industry segment.
A trade publication (often a magazine) is an information source that is intended for a very specific audience and will have content that appeals to that audience. Trade publications are often published directly by industry trade associations. Associated content often comes directly from people within the industry, or from freelance or staff writers that have vast experiences writing for that particular trade.
Professionals often subscribe to trade magazines (e.g. Automation World, Adweek) in order to gather valuable information that will help them achieve something within their industry or field.
Trade publications are a Marketer's secret weapon and can be helpful for tracking recent industry developments, identifying competitors or market opportunities, or even finding industry conferences or "trade shows" where the experts are gathering.
Use the resources on the Industry Research Guide to find relevant trade associations (e.g. Fantasy Sports Trade Associations, Brewers Association).
For additional industry/trade associations, search Google as follows: Industry Description AND association site:org