Definition of 'Trade Deficit'
Parkin and Bade's Economics Second Edition defines trade balance as:

The value of all the goods and services we sell to other countries (exports) minus the value of all the goods and services we buy from foreigners
imports) is called our trade balance
An economic measure of a negative balance of trade in which a country's imports exceeds its exports. A trade deficit represents an outflow 
of domestic currency to foreign markets.

Timeline of trade deficit
1975 - last trade surplus - exports exceeded foreign imports by $12,400 million
1987 - trade deficit swelled to $153,300 million
trade gap sank, dollar depreciated, economic growth in foreign countries increased demand for us exports
late 1990's - deficit swelled again, us economy growing faster than other economies of major trading partners, americans bought more foreign goods
faster than other countries bought american goods. 
1997 - trade deficit $110,000 million, heading higher

Deficits Bad
Economists who consider trade deficits to be bad believe that a nation that consistently runs a current account deficit is borrowing from abroad
or selling off capital assets—long-term assets—to finance current purchases of goods and services. They believe that continual borrowing is not a 
viable long-term strategy, and that selling long-term assets to finance current consumption undermines future production.

Labor unions oppose trade deficits because they believe that when imports exceed exports, jobs are being lost to overseas workers, or soon will be.
On the surface, it seems a reasonable argument, but the data on trade deficits and unemployment don't support it. In the late 1990s, when the trade
deficit reached record highs, unemployment dropped to its lowest level in three decades.

Deficits Good
Economists who consider trade deficits good associate them with positive economic developments, specifically, higher levels of income, consumer
confidence, and investment. They argue that trade deficits enable the United States to import capital to finance investment in productive capacity.
Far from hurting employment, they believe that trade deficits financed by foreign investment in the United States help to boost U.S. employment.

http://www.infoplease.com/cig/economics/trade-deficits-bad-good.html

http://www.investopedia.com/terms/t/trade_deficit.asp

The U.S. trade deficit is when the total goods and services the U.S. imports is greater than the total it exports. In 2012, the total U.S. 
trade deficit was $539.514 billion. This was $2.194 trillion in exports minus $2.734 trillion in imports. This shows the economy is strengthening,
 since it is less than the $559.88 billion deficit in 2011. A lower deficit means exports are starting to gain on imports. This is good for business,
 which will eventually create more U.S.jobs. It's also less than the record $753 billion trade deficit in 2006. 

what drives the deficit?

Petroleum products In 2012, the U.S. imported $313 billion in petroleum-related products, up from $252 billion in 2010. This was despite higher oil
prices, which jumped from an annual average of $74.67 /barrel to $101.16/barrel in two years. Petroleum-related products include crude oil, natural gas,
 fuel oil and other petroleum-based distillates such as kerosene.

consumer products, cars
Drugs, Consumer Electronics, Clothing, Household Goods, and Furniture. In 2012, the U.S. ran a $335 billion deficit in consumer products, importing
$516 billion while only exporting $181 billion. This was higher than prior years, despite a relatively weak dollar and resultant mild inflation.

Why an Ongoing Trade Deficit Weakens the Economy:

An ongoing trade deficit is detrimental to the nation’s economy over the long term because it is financed with debt. In other words, the U.S. can
buy more than it makes because the countries that it buys from are lending it the money. It's like a party where you’ve run out of money, but the
pizza place is willing to keep sending you pizzas and put it on your tab. Of course, this can only go on as long as there are no other customers for
the pizza, and the pizza place can afford to loan you the money. One day the lending countries may decide to ask the U.S. to repay the debt. On that
day, the party is over.

http://useconomy.about.com/od/tradepolicy/p/Trade_Deficit.htm

Effects of a Trade Deficit

Initially, a trade deficit is not a bad thing. It raises the standard of living of a country's residents, since they now have access to a wider variety
of goods and services for a more competitive price. It can reduce the threat of inflation, since the products are priced lower. A trade deficit can
also indicate that the country's residents are feeling confident, and wealthy, enough to buy more than the country produces.

Over time, however, a trade deficit can cause jobs outsourcing. That's because, as a country imports certain goods rather than buying domestically,
the local companies start to go out of business. The domestic business itself will lose the expertise needed to produce that good competitively. As
a result, fewer jobs in that industry are created in the home country. Instead, the foreign companies hire new workers to keep up with the demand for
their exports.