Keyword Planner is a free Google AdWords tool that helps users research keywords, understand keyword competitiveness and search volume, and generate new keyword ideas. It provides data on the number of advertisers bidding on keywords, average monthly searches, and estimated costs-per-click. Users can enter keywords or categories of interest, customize location and language settings, and download keyword data to an Excel file. The tool helps optimize websites by identifying popular but less competitive "long tail" keywords to target and incorporating those keywords throughout the website content and code.