There is no difference in meaning. Both mean exactly the same thing. The only difference is in the level of formality and ‘properness’.
Linguists call this the language register, sometimes also referred to as tenor, tone or style.
Put simply, a register is a variety of a language used in a particular social setting, using certain words, phrases and contractions that are not normally used in other settings (or if they did, they may sound strange or out of place).
For example, when speaking in a formal setting, an English speaker is more likely to use features of prescribed grammar, like pronouncing words ending in -ing with a velar nasal instead of an alveolar nasal (e.g. «walking», not «walkin’»). Or she or he may choose more formal words (e.g. father vs. dad, child vs. kid, and so on). She or he would also usually refrain from using contractions like «ain’t».
It may sound intuitive but the differences between how one may talk on the street, with friends, during a job interview or when writing formal letters and emails are amazing, even though it is the same person speaking.
This is why a register is a language variation defined by use, not user. The same person may use more than one register depending on the context or social setting.
Indeed, the term «register» was first used by the linguist Thomas Bertram Reid in 1956, and popuarised in the 1960s by a group of linguists who wanted to distinguish between language variations according to the user («defined by variables like social background, geography, sex and age») and variations according to how and when the language is being used («in the sense that each speaker has a range of varieties and choices between them at different times»).
Register should therefore be distinguished from other, identity-based types of language variation, such as regional and age dialects, even though it often overlaps with them. It should also be distinguished from jargon, which is technical terminology used for a special activity or by a special group, such as computer geeks, scientists, lawyers and so on.
One of the most analysed areas where the use of language is determined by the situation is the formality scale. The term «register» is often used as shorthand for formal/informal styles. But many would argue that this is a simplistic definition, because register is about more than just formality, as we have already said.
In one prominent model, Martin Joos (1961) describes five styles in spoken English: frozen or static register, formal, consultative or participatory, casual and intimate.
Foreign speakers may find it difficult – especially if they are beginners – to switch between registers. This is called «code-switching» in linguistics. That is why they may sometimes sound like TV presenters or politicians (i.e. very formal), even when speaking to close friends. Or they may sound too informal, even rude, without intending to.
The only solution to this is to familiarise yourself with the social and cultural associations of words and expressions – which is the basis of language registers. Reading and listening to popular culture, interacting with native speakers and so on.
And getting the register right will certainly get you higher marks in oral language tests such Toefl or IELTS!