al-Salamu `Aleykum,
A useful thing I heard in a Hadithi lecture and I shall relate it to you regarding describing some individuals or scholars in the books of Rijal with certain titles.
If a scholar was described as "Musnid", then he is one who's job is to memorize and relate narrations with their chains even if he never understood their content.
If a scholar was described as "Muhaddith", then he is one who memorizes Hadith and has a good understanding of the sciences of Hadith and conditions of narrators.
If a scholar was described as "Hafiz", then he is one who memorizes and understands a great amount of Hadith and excels in their sciences and has deep knowledge on the conditions of the narrators and the various chains for the reports.
Note: When we say "memorize", we don't mean that a person memorizes a book such as Sahih al-Bukhari that contains around 8,000, but we mean memorizing 70,000 like Ishaq bin Rahawayh, or 300,000 narrations like Muslim bin Hajjaj, or 700,000 like abu Zur`ah al-Razi, or 1,000,000 like Ahmad ibn Hanbal.
After the above comes the title of "Hujjah" then after it the title of "Hakim" then finally that of "Ameer-ul-Mu'mineen fil-Hadith" and it is the greatest rank.
The differences between all of these later ranks, is how many narrations does one memorize and know, and of course these people are nonexistent on the ground of reality today, as we don't know anyone who barely qualifies as being a "Hafiz" today. It is also important to note that Muslims today randomly throw these labels at contemporary scholars they love, out of ignorance and extremism, such as "`Allamah" and "Shaykh-ul-Islam" and such, and no person of knowledge takes this seriously but the condition of us laypeople today is regrettable.