Block Statement
Concurrent Statement ---- used in ----> Architecture

Syntax

label: block (optional_guard_condition)
	declarations
begin
	concurrent statements
end block label;
See LRM section 9.1

Rules and Examples

The label is compulsory 
CONTROL_LOGIC: block
begin
  U1: CONTROLLER_A
    port map (CLK,X,Y,Z);
  U2: CONTROLLER_A
      port map (CLK,A,B,C);
end block CONTROL_LOGIC;

DATA_PATH: block
begin
  U3: DATAPATH_A port map
    (BUS_A,BUS_B,BUS_C,Z);
  U4: DATAPATH_B port map
    (BUS_A,BUS_C,BUS_D,C);
end block DATA_PATH;
Without a guard condition a block is a grouping together of concurrent statements within an architecture. It may have local signals, constants etc. declared.
Blocks may contain further blocks, implying a form of hierarchy within a single architecture.
A Block may contain any of the declarations possible for an architecture. Items declared within a block are only visible inside it.
IF an optional guard condition is included, the block becomes a guarded block. the guard condition must return a boolean value, and controls guarded signal assignments within the block. If the guard condition evaluates to false, the drive to any guarded signals from the block is "switched off". Such signals must be declared to be guarded signals of a resolved type. Guarded signals can be declared by adding the words bus or register after the name of the type of the signal. The difference between bus and register signals is that if all drivers to a bus signal are "switched off", it requires a resolution function to provide a value for the signal but a register signal retains its last driven value after all drivers to it have been switched off. 
architecture BLKS of TRISTATE is
  signal INT: std_logic bus;
begin
  DRIVER_1: block (EN = '1')
  begin
    INT <= guarded DATA_1;
  end block DRIVER_1;
end BLKS;

Synthesis Issues

Unguarded block statements are usually ignored by logic synthesis tools (i.i. all blocks within an architecture are "flattened").
Guarded block statements are not usually supported for synthesis.

Sequential (i.e. flip-flop and register) behaviour can be modelled using guarded blocks, but again for synthesis and readability it is better described using "clocked" processes.

Whats New in '93

In VHDL-93 the keyword block (or the guard condition, if there is one), may be followed by the keyword is, for consistancy.:

label: block (optional guard_condition) is
 -- etc