Were dinosaurs 'hot-blooded' or 'cold-blooded'?
This is a question I have often heard and there is no one certain answer to it. There are several scientific hypothesis about the methabolism of dinosaurs, but none of them has been ever proved or completely disproved.

Arts of methabolism

First of all, the term "cold-blooded" and "hot-blooded" are not scientific. In biology the organisms are classified according to how constant their body temperature is:


homeothermic (from greek "hom(e)o" = equal, same) -- organisms, who manage to maintain constant body temperature.
poikilothermic (from greek "poikilos" = changing, changeable) - ogranisms, whose body temperature fluctuates over time. It is implied here, that  the body temperature follows the fluctuation of the environment temperature, even though this is strictly not correct (see below)


There is another orthogonal classification, which takes into account the metabolic properties of the organisms:


endothermic organisms actively produce heat in their bodies, whereas
exotermic organisms mostly rely upon external heat sources.


What do we know about dinosaurs' methabolism?

Unfortunately there is no one living dinosaur, so we can only reason about their methabolism basing upon the fossils (and possible DNA from moskitos trapped into ambers).

One of the most cited works here is the "Dinosaur Fossils Predict Body Temperatures" (link to the free full text), where the authors write:


  Perhaps the greatest mystery surrounding dinosaurs concerns whether
  they were endotherms, ectotherms, or some unique intermediate form.
  Here we present a model that yields estimates of dinosaur body
  temperature based on ontogenetic growth trajectories obtained from
  fossil bones. The model predicts that dinosaur body temperatures
  increased with body mass from approximately 25 °C at 12 kg to
  approximately 41 °C at 13,000 kg. The model also successfully predicts
  observed increases in body temperature with body mass for extant
  crocodiles. These results provide direct evidence that dinosaurs were
  reptiles that exhibited inertial homeothermy.


And exactly here we need the terminology I presented above to understand the findings:


Dinosarier were endothermic -- they were able to actively produce heat in their bodies.
The produced heat can only be taken away through their body surface. Due to the fact that the body surface increases quadratically (n2) with the size, whereas the weight grows cubically (n3), the bigger dinosaurs were the higher temperature their bodies reached. That could be one of the growth limiting factors and explains why especially big species preferred to live in water.
Even though the dinosaries were homeothermic, we don't know anything about their thermoregulation. Modern reptilia do not have the thermoregulation mechanisms that would keep the body temperature constant. Dinosariers, however, due to increased heat production might have less fluctuations in their body temperature, than smaller reptilia. This is termed as "inertial homeothermy" here.