The arrangement of the Seder plate according to the Arizal includes one space for "Marror" and another for "Chazeres" (the latter intended for use in Koreich). Does this reflect a tradition to use lettuce only for Koreich, but some other species for the primary Marror? The names are especially jarring for those who use horseradish exclusively.
Shaar Hakollel (48:11) says that indeed some people, because of this difference in terminology, do use different species for each one. Their rationale, he says, is that they're not sure which are the five species listed in the Mishnah (Pesachim 39a), so they prefer to use an actual bitter vegetable (horseradish, I guess) for maror, and then lettuce for chazeres.
However, he says that this practice is incorrect: lettuce should preferably be used for both. (Present-day Chabad custom is to compromise and use lettuce and horseradish for both.) The reason for calling the stuff for koreich "chazeres," he says, is to make it clear that it needn't be the actual species called "maror" (wormwood).
In his commentary on the Haggadah, the Lubavitcher Rebbe zt"l cites this, but argues that if that's the reason, then the term "chazeres" should have been used for both items on the Seder plate. So he suggests that the reason for the different terms, maror vs. chazeres, is simply that each of them reflects different Kabbalistic concepts: one related to the maror that's eaten alone, the other to koreich - but not necessarily that they are meant to be different species.
The other vegetable is probably the one used for karpas, if I understand the Ramba"m in Hilchos chametz umatza 8:1,2 correctly. It is clear from the g'mara on which these halachos are based (P'sachim 114b) and Tosafos (ad loc.) that chazeres (i.e. some type of lettuce) is the assumed species of maror and a suitable species of karpas as well.