B.Tech : In India its a professional engineering undergraduate degree awarded after completion of 4 years of engineering study.
Diploma : A diploma is a specific academic award usually earned in professional/vocational courses, e.g., Engineering, Pharmacy, Design, etc. Its equivalent to general degree in that discipline for example, Diploma in engineering of Computer Science and engineering is rated differently than Bachelor of engineering in Computer Science and engineering but is equivalent to Bachelor of Science in Computer Science. It is taken soon after completing 10th.
The Bachelor of Technology : abbreviated as B.Tech (engineering) which is commonly a 3-4yrs course and can be done only after completion of higher secondary.
Diploma is a course which can be pursued just after 10th. And is a 3yrs course on which ever subject one wants. (not necessarily engineering subjects).
Diploma is considered as a lower degree than that of b-tech. This comparison can be done only when one have taken engineering subjects to do diploma
One who has completed diploma in engineering subjects can enter b-tech colleges through lateral entry. However they would enter in the 2nd year directly!