Diploma

The duration of a degree course is usually 3 to 4 years and the emphasis is on academics. For example, B.Com (H) is a 3-year degree course that deals with subjects like accountancy, economics and business studies;
Only a university can confer a degree. To pursue higher education, only a degree from a UGC approved university is accepted. AICTE approved private institutes are affiliated to UGC approved universities. In case a person graduates from an AICTE approved private institute affiliated to a UGC approved university, he still gets the degree of the university.
The curriculum is designed in a way that it provides an overview of many subjects, other than the one the student may be interested in. For example, in an engineering degree course for Computer Science, the first year focuses on subjects like Physics, Chemistry, Electrical Engineering, Mechanics, and Engineering Drawing apart from languages like C and COBOL; subsequent years have subjects like Digital Electronics and Analog Electronics. Typically, there is one subject that you major in, called the specialization whereas other subjects are called electives.
After a degree course, you can go for higher education to better your skills or look for a job in your field. For example, after a B.Com (H) degree, you can go for a job in accounting or finance, or go for an MBA or M.Com.
A person must be an HSC or equivalent, to pursue a bachelors degree. One must possess a bachelor degree to pursue a master degree.
A diploma course is a short term course (usually 1 to 2 years in duration) that focuses on training a person in a particular field.
An institute, a polytechnic or even a university can offer a diploma course. There is no need for the institute or the university to be UGC or AICTE approved.
The course is designed in a way such that the person gets basic theoretical knowledge about the course, apart from the extensive practical training needed in the job industry. For this purpose, a lot of diploma courses have on-the-job training and apprenticeship and focus more on practical exams. For example, a diploma in Computers involves subjects centred on programming languages and computer hardware only.
Scope
After a diploma, you can either enroll in a bachelor degree course or look for jobs.