"Expert Advisor" is a term that gets thrown around in MetaTrader circles as if everyone already knows what it means. Strip away the jargon and it's a simple idea: a program that watches a chart and trades on your behalf, following rules you defined in advance - not fundamentally different from setting an alarm clock, just for market conditions instead of time.
What an EA actually is
An Expert Advisor is software - written in MQL5 for MetaTrader 5, or MQL4 for the older MetaTrader 4 - that attaches to a chart and runs continuously, checking your conditions on every new price tick or candle and acting the moment they're met. It doesn't get tired, doesn't hesitate, and doesn't deviate from what it was told, which is both the appeal and the risk: it follows a bad rule exactly as faithfully as a good one.
EA vs. cBot vs. "trading bot"
Different platforms use different names for the same underlying idea. MetaTrader calls it an Expert Advisor. cTrader calls its equivalent a cBot, written in C# instead of MQL. "Trading bot" is just the informal, platform-agnostic term for either one. An MT5 EA and an MT4 EA aren't interchangeable either, despite the shared name - MQL5 and MQL4 are different languages, so a strategy has to be built (or exported) separately for each platform, covered in more depth in MT4 vs MT5 vs cTrader.
What's actually inside one
Strip an EA down and there are really only three parts. A condition that decides when to act - price crossing a moving average, an RSI level, a candle pattern, anything that can be checked automatically. An action that decides what to do once that condition is true - buy, sell, close. And risk logic that decides how much to risk and where the trade exits - position size, stop loss, take profit. Every EA, no matter how sophisticated it looks from the outside, is built from those same three pieces.
You don't have to write one to use one
The traditional path to having your own EA meant learning MQL5 or MQL4 first. That's no longer the only option - a block-based builder lets you define the same three parts (condition, action, risk logic) by connecting pieces visually, then generates the actual .mq5, .mq4, or .cs file for you. Not knowing what a function or a variable is stops mattering; being able to describe the rule in plain language is what actually matters.