A block is one or more lines of code demarcated from its surroundings as having a separate nature or purpose. A block is announced by a line stating what type of block it is; then comes the code of the block; and finally the block is terminated by a line starting with the keyword end. Blocks can occur within blocks.
It's very easy to spot a block in AppleScript code, because in decompiled code its lines are indented from the announcement line and the termination line. For example:
myHandler( ) on myHandler( ) repeat 3 times display dialog "Howdy" end repeat end myHandler
That code contains two blocks. One is announced with the on myHandler line, and is terminated by the end myHandler line; everything in between them is the code of that block. That code consists of another block, announced with the repeat line and terminated by the end repeat line; the line of code in between them is the code of that block.
In this book I frequently refer to such blocks by their announcement keyword; for example, I might say "a repeat block."
Some blocks (just two, actuallytell blocks and if blocks) have single-line variants. This permits some rather twisted condensed syntax. For example:
tell application "Finder" if exists folder "Mannie" then reveal folder "Mannie" end if end tell
You can reduce one or both of those blocks to a single line. So, this is legal (but I never talk this way, and I don't recommend you do either):
tell application "Finder" to if exists folder "Mannie" then reveal folder "Mannie" end if
The only blocks you can make in AppleScript are those for which keywords are supplied; you cannot indent arbitrarily for clarity, as you can in UserTalk or C. So, for example, in UserTalk you can say this:
local (x) bundle x = 4 msg (x)
The keyword bundle here does nothing except to allow some code to be indented for clarity. It also provides a further level of local scope. In AppleScript the scoping issue doesn't arise, but a way of indenting for clarity might still be nice. To achieve it you would need to misuse an existing block type. For example:
local x repeat 1 times set x to 4 end repeat display dialog x