In addition to terdon’s answer,

• “Obviously” -exec … must be terminated with either a semicolon (;) or a plus sign (+).  Semicolon is a special character in the shell (or, at least, every shell I’ve ever used), so, if it is to be used as part of the find command, it must be escaped or quoted (\;, ";", or ';').

• With -exec … ;, the {} string may appear any number of times in the command, including zero, or two or more, in any position.  ^{See this for an example of why you might want to do -exec without using {}.}  Having two or more appearances is useful principally because, in (at least) some versions of find, the {} doesn’t need to be a word by itself; it can have other characters at the beginning or end; e.g.,

    find . -type f -exec mv {} {}.bak ";"
  

With -exec … +, the {} string must appear as the last argument before the +.  A command like

    find . -name "*.bak" -exec mv {} backup_folder +

results in the enigmatic find: missing argument to ‘-exec’ error message.

• A workaround for this that’s specific to the cp and mv commands is

       find . -name "*.bak" -exec mv -t backup_folder {} +
  

  or

       find . -name "*.bak" -exec mv --target-directory=backup_folder {} +
  

The {} must be a word by itself; it cannot have other characters at the beginning or end.  And, in (at least) some versions of find, you may not have more than one {}.

• A sanity note: You can say

find . -name "*.sh" -type f -executable -exec {} optional args here ";"

to run each of your scripts.  But

find . -name "*.sh" -type f -executable -exec {} +

runs one of your scripts, with the names of all the others as parameters.  This is similar to saying

    ./*.sh

as a shell command, except find does not guarantee that it sorts its results, so you aren’t guaranteed of running aaa.sh (your alphabetically first *.sh file) as you would be with running ./*.sh.

• An aspect of find that may not be perfectly clear to beginners is that the command line is, effectively, an executable statement in an arcane language.  For example,

    find . -name "*.sh" -type f -executable -print
  

means

    for each file
        if the file’s name matches `*.sh` (i.e., if it ends with `.sh`)
        then
            if it is a plain file (i.e., not a directory)
            then
                if it is executable (i.e., the appropriate `---x--x--x` bit is set)
                then
                    print the file’s name
                end if
            end if
        end if
    end loop

or, simply,

    for each file
        if the file’s name matches `*.sh`  AND  it is a plain file  AND  it is executable
        then
            print the file’s name
        end if
    end loop

Some of the - keywords are both an executable action and a test.  In particular, this is true for -exec … ;; for example,

    find . -type f -exec grep -q cat {} ";" -print

will print the names of all files containing the string “cat”.  And, while this is something that grep can do by itself (with the -l (lower-case L) option), it can be useful to use it with find to find files that contain a certain string AND have a certain size AND are owned by a certain owner AND were modified in a certain time range, ….

However, this does not work for -exec … +.  Since -exec … + executes one command for multiple files, it doesn’t make sense to use it as a logical condition inside a for each file … loop.

• The flip side of the above is that find generally exits with an exit status of 0 unless you give it invalid arguments or it encounters a directory that it can’t read.  Even if a program that you execute fails (exits with a non-zero exit status), find will exit with an exit status of 0.  Except if a program that you execute with -exec … + fails (exits with a non-zero exit status), find will exit with a non-zero exit status.

In addition to a million versions of find(1) and testing what find actually does on a couple of systems, The Open Group Base Specifications Issue 7, 2013 Edition provided some of the information on what find must, may, and must not do.