During the First Opium War, on the march to Peking, many of the battles that occurred did not involve Chinese soldiers, but fighters that the British described as "Tartars," being vaguely described as either Mongolians or Turks. Who were these men and from where did they come. Were they Mongolians or Turks or both or some others? One thing to note is that the Manchu themselves have sometimes been described as "Tartars," so there is the possibility that they were actually Manchu.
Further research seems to indicate that the last part of the question is correct and that various statements to the effect that the Tartars were "Mongols" or "Turks" may be incorrect. An issue of the United Service Magazine in 1853 published an article stating the following:
The Chinese army contains three very distinct elements: the Tartar Mantchou troops, the regular Chinese troops, and the reserve, wholly Chinese. The first of these bodies is considered the best, and constitutes a sort of Imperial guard, infinitely more devoted than the rest of the army to the defence of the throne and the support of the dynasty, on account of the community of origin, which attaches this elect portion of the army to the sovereign.
The Tartar troops, about 80,000 strong, form eight divisions of 10,000 men each, every division having its own colours. There is a yellow, a white, a red, and a blue flag, and four others containing two of these colours in equal parts. Each military Tartar mandarin belongs, in fact and by title, to the banner under which his ancestors have served; for the Mantchou army has been divided into eight bodies, each rallying round one of these banners, ever since their invasion of China towards the middle of the seventeenth century.
Thus, this would suggest by "Tartar" is simply meant the Manchu.