It's hard to describe an animal accurately enough for a positive ID in
text.
Try a field guide first, since you can look back and forth from the book
to
the animal.  (This author, based in North America, favors the Audubon
guide;
others prefer the Peterson guides for their range maps and
similar-species
sections.  Field guides for Britain and Europe are known to exist, but I
don't know enough about them to make recommendations.)  If you can't
make
a conclusive ID, then post a detailed description of the animal, along
with
any useful information you gathered from the guide ("I thought it might
be
a Flipplezorb's tree frog, but it doesn't have a puce belly").  Someone
will
probably post either a tentative ID or a request for specific
information.

In some cases, the answer to "Can I keep it?" is definitely *no*.  Many
jurisdictions have some form of laws against keeping native wildlife in
captivity, and such laws are sometimes enforced with surprising vigor.
This
is one reason why a positive ID is very important; you don't want to
find
yourself inadvertently violating the law and setting both yourself and
the
animal up for trouble.

Legalities aside, it's often not a good idea to keep animals you find in
the
wild, and you should just release the critter where you found it;
ultimately,
all concerned will probably be happier if you satisfy your herp desires
with
a captive-bred animal.  However, most of us caught garter snakes as kids
and
kept them, and are in no position to take a holier-than-thou stance
against
keeping such animals.  If you want to keep something that crawled out
from
under your azaleas, make sure you've identified it correctly, and *then*
post
asking for care guidelines.  A single posting saying "I don't know what
this
is, but how do I take care of it?" will not get many useful responses.