Charedi is the self-definition of those groups of Orthodox Jews who tend to live in enclosed communities, carefully regulate their interaction with the secular world and strive assiduously to learn Torah and fulfil mitzvot.
Charedi is preferable to the mainstream media's name for such Jews, "ultra-Orthodox," with its implication of having completely gone off the deep end.
Indeed in the mouth of some news outlets, the term "ultra-Orthodox" seems to me to have a whiff of "Der Sturmer" about it.
The word charedi means "those who tremble" or "are afraid." When Isaac discovered that he had blessed Jacob and not Esau, he trembled (charad) very much.
In a phrase that is close to our contemporary usage, Isaiah addresses the God-fearing as "those who tremble (charedim) at His word" (Isaiah 66:5). Charedim there signifies not so much fear, as anxiousness to fulfil God's will.
Although there is an early modern ethical work named Sefer Charedim, it seems that only in the 20th century did the word begin to be used generically to describe a broad swathe of Jewry.
Just as the word "black" has rightly replaced pejorative expressions like "coloured," so too charedi should be used instead of the prejudicial "ultra-Orthodox."