Yes, if someone buys at a particular price, someone else must sell at that price. But they didn't place their orders at the same time. The buy/sell indicates the direction of the filled order. This is needed to make sense of the price.

Imagine an apple exchange where no trades are occurring. Then someone walks in and buys or sells an apple for $1. Do you care whether the person who walked in bought or sold? Well, yes. If the person bought an apple for $1, that means before they walked in, someone was willing to sell an apple for $1 but nobody would buy it. If that person sold an apple for $1, that means before they walked in, someone was willing to buy an apple for $1 but nobody would sell it. So without knowing the direction of the transaction, you can't tell what the price is telling you about the state of the market prior to that transaction.

This is very important in a market that isn't very liquid. For example, say the cheapest anyone is willing to sell a Bitcoin is $1,000, but the most anyone is willing to pay for a Bitcoin is $900. If it stays that way for a while, all the executing buys will be at $1,000 and all the executing sells will be at $900. If you just saw alternating $1,000 and $900 prices without knowing all the buys were at $1,000 and all the sells were at $900, you'd think the price was changing when it's actually constant. Orders that don't execute at all aren't listed because no money changes hands. So the people selling at $1,000 and buying at $900 won't be listed as transactions.

These are orders. You place an order to buy at, say $600, and it is listed on that list as a bit. It will stay there until someone places a ask to sell at $600 or lower.

The spread between them is an indication of liquidity, or how easy it is to make a transaction go through on the site. If they are very close, lots of BTC/USD is likely to be able to be bought/sold quickly.

Another indication you can gain from this is market depth. You can look at the list and figure out how much will the price change if I buy $1000. Say there is a sell order for .1 BTC that costs $100 ($1000 BTC/USD), another with .5 BTC for $505 ($1010 BTC/USD and another with .38 BTC for $395 ($1020 BTC/USD). By spending your $1000 at one time, you will not be able to spend all of it at the current price ($1000 BTC/USD), but will end up buying those .38 BTC for $1020. The more money is necessary to more the price significantly, the deeper the market is and the easier it is to make large transactions.

For more info, look up bid/ask spread and market depth.

It's like any market for anything of value. Like stocks or bonds or coins.

The bid price is the price someone has asked to purchase that item for. The asked price is the price someone is asking to sell the item for.

So, if someone is saying they will sell 1 BTC for $600 USD and someone else says they will buy 1 BTC for $599 USD, these are orders waiting to be fulfilled. As long as nothing changes, these numbers stay the same.

Now, if someone else comes along and sees someone is willing to buy for $599. and they are willing to sell for $599. They put in an order to sell, and they sell the 1 BTC to the buyer for $599. If there is no one else willing to buy for $599 and the next willing buyer wants to buy at $598. Then the bid price lowers to $598. while the ask is still $600.