Tags are strings that are attached to items on a complex widget ([text] or [canvas]), or to groups of bindings (see [bindtags]). An item can have several tags. 
After assigning them, you can perform actions on a tag, which will be done with all the tagged items. 

In [text] widgets, tags are the only way to assign attributes (font, colors, underline..) to text portions.

In [canvas] widgets, tags are a convenient way of grouping items, and configuring whole groups of them in one command (see [Flag signalling] for examples). But items may also be configured individually there (or, put differently, the numerical id assigned to each item also is a unique tag).
([RS])

'''Usage examples:'''
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 package require Tk
 canvas .c
 # On canvases, tags are known only if at least one item has it
 foreach i {1 2 3 4 5} {
     .c create rect [expr $i*10] 0 [expr $i*10+8] 8 -tag box
 } ;# create multiple canvas objects
 .c itemconfig box -fill red ;# to change their color
 .c lower box ;# to hide them behind a background area
 pack .c

 text .t
 # On text widgets, tags persist even without instances
 # You should configure them early, to avoid lengthy reformatting
 .t tag config bold   -font {helvetica 12 bold}
 .t tag config hilite -background orange
 .t insert end Hello
 .t insert end world bold ;# tags come as 4(6,8..)th element
 .t insert end "out there" {bold hilite}
 .t tag config bold -font {times 11 bold}  ;# change and reformat all
 pack .t
----
Note: Use the full names for foreground and background.  
In the tag config context fg means fgstipple and bg means bgstipple.

[[tutorials]]

[[examples in books, including Effective, JO's, ...]]


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"Tagging passages of text or canvas objects
makes it possible to apply formatting to or recognize events from
the whole collection... with a concise syntactic specification...
not heaps of application specific code." 
Jon Fernquest 
[http://lambda.weblogs.com/discuss/msgReader$4442]
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[Category Tutorial]