What is the difference between V-neck/mermaid and round neck/trumpet wedding dresses ?

"V-neck" vs. "Round neck" of the dresses refers to the cut
and shape of the neckline.  V-neck dresses are cut to a point which (the neckline looks
like a "V"), in a wedding dress, it is currently popular to be very low plunging to
where the point ends somewhere along the sternum.  Depending on how low it plunges will
reveal more or less cleavage.

A round neck dress is softer
than a V-neck - the neck is rounded or sometimes called a "scoop neck."  Round neck
dresses can be cut as high as the collar bone and go lower - the lower cut, again, the
more cleavage is revealed.

"Mermaid" vs. "Trumpet" refers
to silhouette of the dress, which means the shape, cut, and how it fits on the body. 
The mermaid and trumpet silhouettes are actually quite similar.  They both are tight and
form-fitting from the chest to about the top of the knees, then billow out at the very
bottom - simulating either a mermaid tail or the horn of the trumpet.  It seems the
biggest difference is that in a "mermaid" cut the piece of fabric that forms the bodice
and 'tail' actually ends (and usually has a hem-line) and other fabric is added to form
the lower piece, or the 'fins.'  Sometimes these are done with multiple
layers.

A "trumpet" silhouette is created with all one
piece of fabric that flows into the same shape but the line from top to bottom is never
broken with a hem.

To describe in general terms, the
difference between these two dresses is that the first one likely has much more angular
lines.  Fashion words used to describe it might include dramatic,
geometric, or linear.  The second dress would
be considered softer (than the princess dress),
elegant, and the piece would move from top to
bottom without any lines to break it up.

Click the links
below to see images of the closest thing I could find.  The "trumpet" dress is not
exactly round neck - but I wouldn't consider it a v-neck either.