A micronation is an entity that claims to be a sovereign government, however lacks any recognition from established sovereign states. There is no current definition of what a "micronation" truly is, as the goals of a micronation often comes down to the ideals of its founder or citizens. For example, many of the more well known micronations like the Principality of Sealand see true recognition and secession as its primary goal (ie. recognition from other established sovereign states). Others base their origin around cultural ideas or controversies with their parent government, as seen with the Royal Republic of Ladonia. A majority of micronations however, act simply more as government simulations, with no true intent on secession from their parent nation.
Often times one might see the term "mockgov" used interchangeably with the term "micronation". While it is true that many micronations post the introduction of the internet have become more like government simulators, there is a key difference. Mockgovs, or mock governments do not attempt to forge their own identity. Many mockgovs imitate already existing or previously existing soverign governments, without any sort of deviation. Often, these mockgovs claim unreasonably large portions of land, and take place either entirely online, or one video games, such as Minecraft. Mockgov is used as a derogatory term, alongside "LARP" and "role-play" to describe micronations that often partake in unrealistic goals, which make them seem more like mockgov simulators, rather than actual sovereign entities.
For more information on micronations, check:
Microwiki- The most relevant and popular wikipedia site for micronations and related topics. Plenty of information, barring some censorship issues and outdated topics.
Wikipedia- Has a limited list of the most well known micronations.
Republic of Molossia- Has some information regarding micronations.