What is hujjat? Who is hujjat of Allah now?

Is this what 'hujjat' is? > in every era there must be one person to guide people and to help them obtain spiritual growth and fulfill their responsibilities towards their society. Who is 'hujjat' now? Is Imam Mahdi Allah's hujjat now? And a similar question, what does "Wali Faqih (ولایة الفقیه)" mean? Is it something same like Hojjat or 'different?' If different, who is 'Wali Faqih (ولایة الفقیه)' now? *(Shia viewpoint only)*