Online trading can only be accessed via an electronic trading platform that a broker will supply to you. Most trading platforms provided by brokers are free of charge, but some come with a monthly/annual price tag attached to them.
Usually, the free trading platforms usually only cover the basic needs complemented by more advanced features. MetaTrader 4, MetaTrader 5 and cTrader are among the most popular free trading platforms.
On the other hand, a paid trading platform will cover proprietary trading tools and more complex features tailored for all trading styles. The only downside with a paid platform is that traders need to pay for the license to use the platform. Some of the most popular paid platforms are TradingView, NinjaTrader and Trade Station.