I only know about one exchange for sure but I'm pretty confident this is a widespread concept and applies to many other exchanges, too.
https://www.bitcoin.de uses candlestick charts. These charts exist in traditional markets, too. There, a candle stick typically represents 1 day. That day has among others an opening and a closing price. Note that the closing price December 29th doesn't have to be the opening price of December 30th and the opening price of December 29th wasn't necessarily the closing price of December 28th, in such markets. In fact, it's fairly unlikely that these prices are the same.
https://www.bitcoin.de always trades. Its candlesticks are different.
First of all, they have different sizes depending on the length of the period of time the diagram shows. In the 48 h chart, they only are 15 minutes wide. In the 1 Y chart, they are as wide as in old markets: 1 day. In the 5 Y chart, they are a week wide.
Furthermore, the closing price of one candlestick is always the same as the opening price of the next one. This is because they simply slice time up on the clock (00:00, 00:15, 00:30, etc.) or calendar (December 29, December 30, December 31th, etc.). Every time slice's opening price is the price when that time slice started and its closing price is the price when it ended. Because the one time slice begins right when the one before it ends and price is is simplified to a function of time, the opening price of a slice must always be the same as the closing price of the previous one.