From the Pali Text Society Dictionary:
Taṇhā (f.) [Sk. tṛṣṇā, besides tarśa (m.) & ṭṛṣ (f.)=Av. tarśna thirst, Gr. tarsi/a dryness, Goth. paúrsus, Ohg. durst, E. drought & thirst; to *ters to be, or to make dry in Gr. te/rsomai, Lat. torreo to roast, Goth. gapaírsan, Ohg. derren. -- Another form of t. is tasiṇā] lit. drought, thirst; fig. craving, hunger for, excitement, the fever of unsatisfied longing (c. loc.: kabaḷinkāre āhāre "thirst" for solid food S ii.101 sq.; cīvare piṇḍapāte taṇhā=greed for Sn 339). Oppd to peace of mind (upekhā, santi). -- A. Literal meaning: khudāya taṇhāya ca khajjamānā tormented by hunger & thirst Pv ii.15 (=pipāsāya PvA 69). -- B. In its secondary meaning: taṇhā is a state of mind that leads to rebirth. ..."
It's very long, this is just a snippet.
Lobha [cp. Vedic & Epic Sk. lobha; fr. lubh: see lub- bhati] covetousness, greed. Defined at Vism 468 as "lubbhanti tena, sayaŋ vā lubbhati, lubbhana -- mattam eva vā taŋ," with several comparisons following. <-> Often found in triad of lobha, dosa, moha (greed, anger, bewilderment, forming the three principles of demerit: see kusala -- mūla), e. g. at A iv.96; It 83, 84; Vism 116; Dukp 9, 18 sq. See dosa & moha. -- D iii.214, 275; S i.16, 43, 63, 123 (bhava˚); v.88; A i.64 (˚kkhaya), 160 (visama˚), cp. D iii.70 sq.; ii.67; Sn 367, 371, 537 (˚kodha), 663, 706, 864, 941 (˚pāpa); Nd1 15, 16, 261; J iv.11 (kodha, dosa, l.); Dhs 982, 1059; Vbh 208, 341, 381, 402; Nett 13, 27; Vism 103; VbhA 18; PvA 7, 13, 17, 89 (+dosa), 102; VvA 14; Sdhp 52 (˚moha), 266. -- alobha disinterestedness D iii.214; Dhs 32.
-- dhammā (pl.) affection of greed, things belonging to greed; (adj.) (of) greedy character M i.91; iii.37; D i.224, 230; S iv.111; A iii.350; J iv.11. -- mūla the root of greed Vism 454 (eightfold; with dosa -- mūla & moha -- mūla)."http://dsal.uchicago.edu/dictionaries/pali/
(Note: You must use correct romanized Pali to enter words with the proper accent symbols like above.)
My opinion: While I think the two are related, it may differ by its use in context. Taṇhā is a much more fundamental concept: an 'unsatisfied longing', associated with a feeling (maybe unpleasant) which is viewed as suffering. Lobha is in regard to a beings disposition or character: i.e. one *who wants more than what he needs (is required)*, or wants what he does not have or what others have. From English dictionary: "avarice; reprehensible acquisitiveness."