In England, Wales and Northern Ireland the HND is a BTEC qualification awarded by Edexcel and in Scotland is a Higher National awarded by the Scottish Qualifications Authority. The attainment level is roughly equivalent to second year of university, a Diploma of Higher Education but in some cases may be marginally below that of a bachelor's degree. An HND takes two years of full-time study, or one year full-time following successful completion of a Higher National Certificate; part-time study takes longer.
In Scotland an HND is Level 8 on the Scottish Credit and Qualifications Framework and in England, Wales and Northern Ireland it is Level 5 on the National Qualifications Framework. It is quite common for those who have achieved a HND to add to their qualification by progressing to other levels such as professional qualifications, or a degree. It's graded similar to GNVQ - modules are graded, rather than the HND award itself - Pass, Merit and Distinction with Distinction the highest grade and Pass the lowest grade.
Many universities will take students who have completed their HND onto either level 2 [2nd year] or level 3 [3rd year] of a degree course. [This is often called a 'top-up.] Usually which of these years depends on the modules taken in the HND. It also means that after three years [or four if, say, a business placement year is taken] a student could have both the HND and an honours degree if studying in a UK university excluding Scotland. Scottish Honour Degrees are normally 4 year courses, and so an extra year of study is required.