Diffenece between tab and cat.

First of all what we are talking about here is the select query that returns tables that exist in the database.

Hence what we are differentiating here is
select * from tab and
select * from cat

Well I tried a lot to search for the difference by could not find anywhere, a proper answer.

Anyway, main difference is primarily concerned about the cluster in oracle databases.
Lets see it practically to make out the difference, observe the commands and there out put when they are run

desc tab;
output shows that there are 3 columns named
1:)table name
2:)table type
3:)cluster id

however,
desc cat;
output shows that there are 2 columns named
1:)table name
2:)table type

Hence the main difference is that with the cluster-id. Now whats that cluster id thing?
Well in order to increase the scalability of oracle databases ,Oracle came up with a concept called Oracle Clusters, now using these clusters u can creating multiple instances of oracle database on different machines.

And hence the additional column is nothing about talking about this cluster id. Now the difference b/t those select commands are quit simple.