"What does it feel like?" many people have asked me this well
here you go:
This depends on which way your mood has swung.

Depression

The feeling of depression is something we all experience from time to time. It can even help us to recognise and deal with problems in our lives but in clinical depression or bipolar disorder, the feeling of depression is worse. It goes on for longer and makes it difficult or impossible to deal with the normal things of life. If you become depressed, you will notice some of these changes:

Emotional:

feelings of unhappiness that don't go away
feeling that you want to burst into tears for no reason
losing interest in things
being unable to enjoy things
feeling restless and agitated
losing self-confidence
feeling useless, inadequate and hopeless
feeling more irritable than usual
thinking of suicide.
Thinking:

can’t think positively or hopefully
finding it hard to make even simple decisions
difficulty in concentrating.
Physical:

losing appetite and weight
difficulty in getting to sleep
waking earlier than usual
feeling utterly tired
constipation
going off sex.
Behaviour:

difficulty in starting or completing things – even everyday chores
crying a lot – or feeling like you want to cry, but not being able to
avoiding contact with other people.
Mania:

Mania is an extreme sense of well-being, energy and optimism. It can be so intense that it affects your thinking and judgement. You may believe strange things about yourself, make bad decisions, and behave in embarrassing, harmful and - occasionally - dangerous ways.
Like depression, it can make it difficult or impossible to deal with life in an effective way. A period of mania can affect both relationships and work. When it isn't so extreme, it is called 'hypomania'.
If you become manic, you may notice that you are:

Emotional:

very happy and excited
irritated with other people who don't share your optimistic outlook
feeling more important than usual.
Thinking:

full of new and exciting ideas
moving quickly from one idea to another
hearing voices that other people can't hear.
Physical:

full of energy
unable or unwilling to sleep
more interested in sex.
Behaviour:

making plans that are grandiose and unrealistic
very active, moving around very quickly
behaving unusually
talking very quickly - other people may find it hard to understand what you are talking about
making odd decisions on the spur of the moment, sometimes with disastrous consequences
recklessly spending your money
over-familiar or recklessly critical with other people
less inhibited in general.
If you are in the middle of a manic episode for the first time, you may not realise that there is anything wrong – although your friends, family or colleagues will. You may even feel offended if someone tries to point this out to you. You increasingly lose touch with day-to-day issues – and with other people's feelings.