Short answers, pulled from the story.
Mjölnir is the hammer of Thor, the Norse god of thunder. It functioned both as a devastating weapon used to slay giants and as a sacred instrument for blessings, consecrations, and healing. The hammer is attested in sources including the 11th century Kvinneby amulet, the Poetic Edda, and the Prose Edda.
The etymology of Mjölnir is disputed. Proposed derivations connect it to words meaning "lightning" in Old Church Slavonic and Russian, to Old Norse words meaning "new snow" or "white" yielding the sense "shining lightning weapon", or to Old Norse and Gothic roots meaning "to grind" yielding "the grinder".
The hammer was forged by the dwarf Eitri while his brother Brokkr worked the bellows. A fly bit Brokkr on the eyelid during the final stage, causing blood to obscure his vision and making him flinch. As a result, Mjölnir was pulled from the forge with a shorter handle than intended. The assembled gods Odin, Thor, and Freyr still judged it the best of all objects presented to them.
Around 1,000 hammer pendants have been unearthed across what are now the Nordic countries, England, northern Germany, the Baltic countries, and Russia. Most are simple pieces in iron or silver; around 100 have more ornate designs. As of 2004, ten percent of all finds came from graves, nearly all of which were cremations.
According to both the eddic poem Vafþrúðnismál and Gylfaginning in the Prose Edda, Thor's sons Móði and Magni survive Ragnarök and return to Asgard holding their father's hammer Mjölnir. The 9th century skald Bragi Boddason and 10th century skald Þjóðólfr of Hvinir also reference the hammer in surviving skaldic verse.
Yes. Mjölnir is the most common sacred artefact among adherents of Heathenry, a modern religious movement, according to scholars Jenny Blain and Robert J. Wallis writing in 2006. In the United States, the National Cemetery Administration offers it as religious emblem number 55, titled "Hammer of Thor", for use on military veteran grave stones.