Lists

set list_name "To Do List"
# Create an Empty list using the list command. Not very nessary  
set to_do [list]
#Add items to our to do list
lappend to_do "Buy Groceries"
lappend to_do "Tame Nature"
lappend to_do "Split Atom"

#Display all things
label .desc -text "The $list_name\n$to_do"
label .one -text "First thing to do : [lindex $to_do 0]" ;#Get first element
label .second -text "Second thing to do : [lindex $to_do 1]";#Get second element
label .third -text "Third thing to do : [lindex $to_do 2]" ;#and so on
pack .desc .one .second .third

#Insert a new Item in the second place
set to_do [linsert $to_do 1 "Capture Osama Bin Laden"]
label .list -text "New List\n$to_do"

#Change an item
set to_do [lreplace $to_do 3 3 "Someone already did that"]
label .latest -text "Latest List\n$to_do"
label .total -text "Total number of jobs to do = [llength $to_do]"
pack .list .latest .total

There are more list related commands like lsort, lset, lrange, lsearch and even others like split, join etc. See the manual for details on these commands.

split
Split is a very useful command for creating lists from stings. If you want to split a string into several items at any character in the sting you can use the... em... split command. If you want to get every word into a list in a sentence this command is very useful.
set words [split $sentence " "]
Here the variable called $sentence is cut at each and every instance of space in it - essentially splitting it into words. Now there is a new list called $words. The result of [lindex $words 0] will the first word, 1 will be second and so on. Another useful thing one can do with this command is to read a file with the read command and split the resulting sting with "\n" to get a list of all lines in the file. You need to get to File Handling to learn that.

Arrays

One can make another kind of array in Tcl with the "array" command. This gives each value in the array a name and the values can be accessed with this name.

#Make the array
array set star_trek {
	location	"Bridge of the Enterprise"
	problem		"Power surge"
	solution	"a fuse"
}
#Access the variables
label .sol1 -text "The $star_trek(problem) at $star_trek(location) \
	could have been avoided if they had $star_trek(solution)."
#Change the variable
set star_trek(solution)	"Wesley"
#Access the changed variables
label .sol2 -text "The $star_trek(problem) at $star_trek(location) \
	could have been avoided if they had $star_trek(solution)."
#Show the solutions
pack .sol1 .sol2

The command 'array set star_trek' creates the array and gives the following arguments as its contents. It can be access with this code
$<ARRAY_NAME>(<ELEMENT_ID>)
It could be changed just as easily by removing the '$' sign as when accessing any other variable. Eg:
set <ARRAY_NAME>(<ELEMENT_ID>) <VALUE>.