define a trade at market

since Forex only has BID and ASK. What would define as trade at market. is it buy at Bid and Sell at ask?
the other issue that i have is that many charting tools use BID as a close price for Forex charts. Would this not skew some of the price moves when i look at historical charting?

You Buy at Ask (the higher one) and Sell at Bid (the lower one). The fact that the chart's Close is the Bid doesn't matter much unless you are testing some scalping strategy.