without commas:
The building where we do most of our work is located downtown.
with commas:
The building, where we do most of our work, is located downtown.
what is the difference between the two in meaning?
,or are they same?
Commas are needed when you have a non-defining relative clause, which gives extra information about the noun.
The building, where we do most of our work, is located downtown.
The clause where we do most of our work is extra information, something we don't need to know. We can remove it along with the commas:
The building is located downtown.
In most cases, this is what we can an aside, or a side comment.
Aside
NOUN
- a remark that is not directly related to the main topic of discussion
Now, when you take context into account, the word downtown is mentioned, which tends to be the busiest place on the map. The implication is that one should expect the office to be loud, full of co-workers moving to and fro, and to bring money, because they will need to pay for parking.
The commas change the meaning of the sentence slightly.
If the phrase set off by the commas is "extra information" as another answer has said, then the sentence could be telling the listener about the location of the building as opposed to the location of something else. "We have several properties; the wharf on the river is only used when our ship has come in, and the park in the county is used for relaxation. The building, where we do most of our work, is downtown."
But the sentence could also be distinguishing among several buildings, and in that case the clause is essential and should not be set off with commas. "We have an office close to the courthouse, and an old farmhouse that we use for some kinds of retreats. The building where we do most of our work is downtown."