A parasitic plant is one that derives some or all of its sustenance from another plant. About 4,100 species in approximately 19 families of flowering plants are known. Parasitic plants have a modified root, the haustorium, that penetrates the host plant and connects to the xylem, phloem, or both. Parasitic plants are characterized as follows:
1a. Obligate parasite – a parasite that cannot complete its life cycle without a host.
1b. Facultative parasite – a parasite that can complete its life cycle independent of a host.
2a. Stem parasite – a parasite that attaches to the host stem.
2b. Root parasite – a parasite that attaches to the host root.
3a. Holoparasite – a plant that is completely parasitic on other plants and has virtually no chlorophyll.
3b. Hemiparasite – a plant that is parasitic under natural conditions and is also photosynthetic to some degree. Hemiparasites may just obtain water and mineral nutrients from the host plant. Many obtain at least part of their organic nutrients from the host as well.
For hemiparasites, one from each of the three sets of terms can be applied to the same species, e.g.
Nuytsia floribunda is an obligate root hemiparasite.
Rhinanthus (Yellow rattle) is a facultative root hemiparasite.
Some parasitic plants are generalists and parasitize many different species, even several different species at once. Dodder (Cuscuta spp., Cassytha spp.) and red rattle (Odontites verna) are generalist parasites. Other parasitic plants are specialists that parasitize a few or even just one species. Beech drops (Epifagus virginiana) is a root holoparasite only on American Beech (Fagus grandifolia). Rafflesia is a holoparasite on the vine Tetrastigma.
Witchweed, broomrape and dodder cause huge economic losses in a variety of herbaceous crops. Mistletoes cause economic damage to forest and ornamental trees.
Mistletoe is a popular Christmas decoration. Kissing under the mistletoe is a Christmas tradition.
Rafflesia arnoldii produces the world's largest flowers at about one meter in diameter. It is a tourist attraction in its native habitat.
A few other parasitic plants are occasionally cultivated for their attractive flowers, such as Nutysia and broomrape.
Parasitic plants are important in research, especially on the loss of photosynthesis during evolution.
A few dozen parasitic plants have occasionally been used as food by people.
Western Australian Christmas tree (Nuytsia floribunda) sometimes damages underground cables. It mistakes the cables for host roots and tries to parasitize them using its sclerenchymatic guillotine 
Plants parasitic on fungi
About 400 species of flowering plants and one gymnosperm (Parasitaxus usta), are parasitic on mycorrhizal fungi. They are termed myco-heterotrophs rather than parasitic plants. Some myco-heterotrophs are Indian pipe (Monotropa uniflora), snow plant (Sarcodes sanguinea), underground orchid (Rhizanthella gardneri), bird's nest orchid (Neottia nidus-avis) and sugarstick (Allotropa virgata).
^ Nickrent, D. L. and Musselman, L. J. 2004. Introduction to Parasitic Flowering Plants. The Plant Health Instructor. DOI: 10.1094/PHI-I-2004-0330-01 
^ Sclerenchymatic guillotine in the haustorium of Nuytsia floribunda 
Digital Atlas of Cuscuta (Convolvulaceae)
The Parasitic Plant Connection
The Strange and Wonderful Myco-heterotrophs
Parasitic Flowering Plants
The Mistletoe Center
Parasitic Plants Biology Study Guide
Nickrent, Daniel L. 2002. Parasitic plants of the world.
Calladine, Ainsley and Pate, John S. 2000. Haustorial structure and functioning of the root hemiparastic tree Nuytsia floribunda (Labill.) R.Br. and water relationships with its hosts. Annals of Botany 85: 723-731.
Milius, Susan. 2000. Botany under the mistletoe: Twisters, spitters, and other flowery thoughts for romantic moments. Science News 158: 411.
Hibberd, Julian M. and Jeschke, W. Dieter. 2001. Solute flux into parasitic plants. Journal of Experimental Botany 52: 2043-2049.