This is a complex area and is governed by many different pieces of legislation - including some very new ones. Please note that all of this legislation, while frequently international in nature, is applied by legislation in the countries involved. The interpretations below apply to how that legislation is applied in the UK. Other countries may differ somewhat.
Please note that these interpretations are very broad-brush and are intended only as a simple reminder to ALWAYS investigate and establish what legislation may apply, and in what manner to you and the animals you wish to keep.
Of particular importance are:-
This serves to protect European Protected Species (EPS). Again, so far as herps are concerned, the vast majority of species are covered by this legislation. As seems inevitably to be the case, already complex EU legislation becomes almost incomprehensible in its UK application.
In essence however, it is illegal to possess or trade in wild-caught EPS unless specifically licensed to do so. The key-phrase here is "wild-caught". It is NOT illegal to possess or trade in captive bred animals, or animals that were acquired before the implementation of the EU legislation (1994). There is a further anomaly in that the UK legislation was not implemented until 2007. As a result, a period of grace was allowed for people to apply for licenses for animals acquired legally (under the UK law THEN prevailing) after 1994. Another quirk in the exceptions is that an EPS which originated outside the EU is also exempted (for example, Lacerta viridis from Russia).
It should be noted, however, that the burden of proof in all cases rests with the owner of the animal. It is not currently clear what would be acceptable as proof, although DEFRA have made it quite clear that they have no intention or desire to penalise people who are keeping animals properly acquired. Fortunately, they and Natural England (who share the same view) are the enforcement agencies rather than the RSPCA who are totally incompetent to deal with such matters, or, indeed, with reptiles and amphibians. Should such a case ever reach court it is likely that the court would accept any "reasonable" proof given the circumstances.
EPS can only be kept or traded if:-
they are the subject of a specifically issued license OR
they were legally acquired prior to 1994 OR
they originated outside the EU OR
they are captive bred.
Detailed information about this new legislation within the UK can be found by following the links pertaining to your own country within the UK at http://www.herpconstrust.org.uk/policy_legal.htm.
Quite simply, in the UK it is illegal to keep a Dangerous Wild Animal without having one's facilities inspected and, if they are adequate, being issued a license.
This legislation provides added protection within the UK for our species. Under this legislation ALL British (originating) herps are protected from trade. Most are protected from "actions .... causing death or injury" and the rarest are protected from habitat destruction or even disturbance (such as may be caused by photography) in the wild.
Last, but far from least, it is illegal to release an alien species into the wild, or to allow it to escape, or to release any of our endangered species into the wild. Alien species can both represent a direct threat to our native animals and they and, even native ones released from captivity, may, without extensive precautions, introduce alien pathogens which could lead to widespread losses.
All of these laws exist for good reason, the protection of the animal themselves in their natural state and habitat and the protection of their habitat. Even if you have no respect for the law, as an animal keeper and lover, have respect for the animals and our native wildlife.
There are several references above to "captive-bred" animals being exempt. Generally, the legal definition of captive bred is "animals that are the offspring of animals which were themselves born or hatched in captivity". Thus, if already gravid animals are acquired, their young, even though born in captivity, were conceived in the wild and are thus not captive-bred. Clearly, however, if THOSE young animals, once ,mature, mate and breed in captivity then their offspring ARE captive bred.