Traditionally, a newspaper recorded the news in an objective way, seeking to offer only the facts, or when the facts were in dispute, offer an even coverage of both sides. This ensured that the public were as informed as they could be in order to make their own judgements. The original intention of newspapers were to keep the populace knowledgeable. More educated, knowledgable populations should be more effective and enlightened members of society.
Tabloids were formed to take on a more sensationalist approach to news. Most often, the focus is less on events and more on people, forming or reinforcing the notion of celebrity. The tabloid's goal is to make money by providing salacious details that feed the population's desire for the darker sides of people in the spotlight. In order to build up fervour in the readership, and as a result of the owners of these tabloids often exerting their influence over the readership, strong, one-sided opinion is used to sway the reader in a single direction.
One style of reporting seeks to lift the level of our knowledge and understanding, preferring truth as pure as possible; the other maintains base understanding and prefers falsities if they support a favourable outcome for the owner.
I give this basic explanation because I worry that enough people don't necessarily know the difference. The apparent most-read newspaper in Australia is the Herald Sun. They say they are a newspaper. It's more often alleged they are a tabloid. To confuse the two is dangerous, especially if you value truth (or if you don't want to be accused of being intelligent, as seems to be a genuine desire). This is the front page of the Herald Sun from a few days back. Which do you think it is?:
