Imam in Arabic means leader. Leader can be pious like the prophets and their successors mentioned in Quran; (We made them Leaders (Imams) guiding people by Our order.) (Sura Anbiyaa', Verse 73). Leader can be bad person who misleads his followers away from Allah and leads them to hellfire as we read in Quran (And We made them leaders call to the Hell Fire) ( Qasas, Verse 41).
The leader in prayers is called in Arabic Imam al-Jama'ah, while the leader of Friday prayers is called Imam al-Jum'ah.
In some non Arabic languages they call him Pesh Namaz (Leader of prayer).
It also depends on the society as we see in Europe and America, they call him Imam , while in other countries they call him Shaikh or Shaykh. In the Indian subcontinent, Sunni and Shia Muslims use the title of Maulana for religious teachers and leaders of prayers.
Shia Muslims live all over the world and they have their own cultural varieties which is respected in Islam as far as it does not contradict any Islamic teaching.