How to read an exchange order book? I am new to trading. On the left side is the order book and on the right, the trade history.

I can see that the order book keeps updating. The number in the market size column increases and decreases, but I don't see any update in the trade history. May I know why? Does the trade history only reflect part of all trades?
Irrespective of bullish or bearish outlook, there are two attitudes a trader can have in the market: active or passive.

Limit orders and stop orders are "passive" trades because they don't move market price. These types of orders are offers to buy or sell bitcoin at a certain price. Passive orders are placed in the exchange order book and remain waiting there until cancelled (by the trader) or consumed by another trader.

Market orders are "active" trades because they consume limit and stop orders in the order book. A market order is executed immediately and moves market price ("last" price) to the level of the last order it consumes. Market orders are not shown in the passive order book, instead they are shown in the trade history column to indicate market activity

Example

Three traders: A, B and C. Market price is at $1,200.

A wants to sell 1 BTC at $1,201 and places a limit sell order in the order book and waits.

B wants to sell 0.5 BTC at $1,202 and also places a limit sell order and waits.

C wants to buy 1.4 BTC as soon as possible at any price near market price. Because C doesn't want to wait, C executes a market order for 1.4 BTC. It consumes A's order for 1 BTC and consumes 0.4 BTC of B's order at $1,202.

After execution of C's buy market order:


last price moves to $1,202
C has bought 1.4 BTC
A has sold 1 BTC
B has sold 0.4 BTC


C's market order for 1.4 BTC is entered in the trade history as the latest buy.

A's order is removed from the order book since it has been "filled". 

The remaining (unconsumed) 0.1 BTC of B's "partially filled" limit sell order remains in the order book until another buyer consumes it, or until B cancels the order.

Summary

Traders who intend to buy or sell at specific price levels place their offers to buy/sell in the order book as passive orders. Active buyers and sellers consume passive orders via immediate market orders. Market orders move price and are entered in the exchange's trade history.

"Market size" is the total of all passive orders waiting in the order book. As traders place and cancel passive orders, market size changes.

"Trade volume" is the total of all active trades (market orders as reflected in the trade history) for a particular period of time.