There are thousands of options available in the United States stock market in which an investor can invest his money. There are more than twenty thousand publicly traded companies in the stock market in the US. Therefore, it becomes a very difficult task for any investor either a beginner or an experienced one to decide which company to invest in. It is impossible to know the names of all companies then how can it be possible to analyze all of them.So, the question arises, what do people use to decide where to invest and which companies to ignore? The answer is usually derived with something called a stock screener. A stock screener is nothing but a tool which is used by investors to short list the companies which are of their interest and where they can invest their hard earned money. Stock screening is a process of shortlist companies which have the potential to be in your portfolio. This screening is done based on the parameters provided by the investor himself.How the share screener works is very simple. Behind a screener is a huge database which has data about all the publicly traded companies. It contains even the historical data which goes past many years and decades. The investor would log on to the screener and give the parameters which interest him to shortlist the companies. The stock screeners act as search engines and bring out the list of companies which match the criteria provided by the investor. These criteria can be anything like minimum market capitalization, minimum revenue of the company, a particular sector, P/E ratio, profit margin etc.There are various stock screeners available in the market today. Most of them are online tools therefore, you just need a reliable and fast internet connection and you can download the stock screener to your computer. The screener makes use of the back end database and then provides the list of companies as per the criteria given as input.Some of the most popular screeners are available online on websites such as Yahoo Finance stock screener, MSN Money website and the Morning Star internet site. And the good part is that all these screeners are absolutely free of cost. There are basically two types of screeners – basic screener and customizable screener. For a beginner in stock market, the basic screener is sufficient while the customizable stock screener is often used by expert and experienced investors in the stock market.