A stock trading simulator is an online tool that allows users to simulate buying and selling stocks and track their virtual performance. It can be a useful tool for practicing the skills of trading, but isn't necessarily realistic, as people inevitably act differently when real money is at stake. While some simulators are largely designed as a fun game, potential serious investors need to look for a simulator that is as realistically as possible, taking into account the markets it models, the trading techniques it simulates, and the frequency and detail of updates.
The first concern when choosing a stock trading simulator is price. Both free and paid-for options are available. Whether to pay for a simulator is a personal choice, but the general principle should be to only pay where doing so brings clear advantages, but to only use free options that still provide a useful service.
It's also important to check that a stock trading simulator covers the type of investments you are interested in. First, you should check that a simulator covers the markets you are considering trading in for real, and that it either covers all stocks on the market, or at least a wide and representative sample. Then, you should check that it covers your preferred trading style. Some simulators only cover straightforward buying and selling, while others allow you to simulate shorting a stock, a technique by which you make a profit if the price falls.
The most important element of a stock trading simulator for somebody considering real trading is that it allows real-time trading, or something close to it. Some more casual simulators are designed basically as games, and only update prices once a day. This may be sufficient for getting to grips with the basics of stock investing, but won't be enough if you are planning serious investing, in particular day trading, which is the strategy of buying and selling stocks on the same day with the aim of making small but quick returns. You may also want to look for a simulator that includes elements designed to be more realistic about costs. This can include taking account of the commission you would have to pay brokers if you traded for real.
A good stock market simulator will allow you to track your performance in terms other than the pure profit or loss. For example, a simulator could compare your performance with that of the market as a whole, which gives a better idea of how your decision making paid off. Some simulators even rank "investors" in a league table. This can be a bit of fun, but shouldn't be taken too seriously as many of the people on it may have been using the simulator for entertainment and won't necessarily have the level of skill of real traders.