This, That, These, Those

Demonstratives are used to point out a particular item. They are as follows:
This (indicates something close to the speaker)
That (indicates something away from the speaker)
These (indicates some things close to the speaker)
Those (indicates some things away from the speaker)

Example:

A: What is this? (pointing to something held in the speaker's hand or near the speaker)
B: A pencil.
A: What is that? (pointing to something at a distance from the speaker)
B: A tree.
Remember that demonstratives are used in reference to the speaker. (What is near "you" may not be near "me" and vice versa.)

A: What's that you're holding?
B: This is an egg. Here, catch!
A: Oops.
B: That's a mess.
A: What are these (holding up a pair of slippers)
B: Those are slippers.

Be sure to use this/that with singular and non-count nouns and these/those with count nouns.

This orange is sour.		These oranges are sweet.
That truck is full.		Those trucks are empty.
This milk is fresh.

Sometimes demonstratives can be used as pronouns (to refer to a particular noun.)

Give me that!		Whose are these?
Give me that (spoon).		Whose are these (socks)?

This and that can also be used with one. However, these and those are more commonly used alone.

I want this one.		I want these. (not these ones)
I'll take that one.		I'll take those.

On the telephone, this and that are used differently in British and American English to identify callers.

Example:

Receiver:		Hello.
Caller:		I was wondering if you would be interested in buying . . .
Receiver:		Who is this? (American)
		Who is that? (British)
		(Both expressions are used to ask the caller to identify himself/herself.)
Caller:		This is Mr. Fuller from the Acme Brush Company.
		(Same for both British and American English)
Caller:		Hi Jack. Long time no see!
Receiver:		Is this Bob? What a surprise! (American)
		Is that Bob? What a surprise! (British)