In movies and news, you often see market charts displayed as lines showing a price as it changes over time like figure 1.1.
Many analysts would rather look at a section of time as one unit. That is to say, they want to see each hour as one separate chunk, for example. So, for that time period, they would want to see what the first price was, how high and low it went, and then finally what the last price was. These values are called the open, high, low, and close, respectively. If you squish that information into one drawing, you get something like figure 1.2.
That is called an OHLC bar for Open, High, Low, Close. Since some analysts like to see the difference between the first and the last price more clearly, they would rather have the section between them thicker. That makes it look like figure 1.3.
You can see why that would be called a candlestick. You have the body of the candle with a wick protruding from the top and/or bottom. So, now you have 3 options to view your charts with: Line, OHLC bar, and Candlestick. You can see these in figure 1.4.
Usually, the candlestick bodies will be colored different for up and down candlesticks. If it closed higher than it opened, maybe it's green, and if it closed lower than it opened, maybe it's red. Of course, you can set them to be whatever you want. Since a candlestick is information for a specific time period, you can choose to view different timeframes of the same chart, too: 1 minute, 1 hour, 1 day, etc.
There are several patterns, setups, and tactics based on candlestick patterns. Some of them rely simply on the shapes that 2 or 3 specific candles make. Many are based on the closing prices of candles with regard to support and resistance zones. You will see candles present in many of the other glossary terms.