As a first step please take some time out to take a look at this forum thread How to Apply For a Fiance(e) Visa

Hopefully it will help your overall basic understanding.

Concerning the specific questions you raised:-

1. The 'Partner' visa's leading to settlement in the UK ALL require the sponsor to comply with the relevant income threshold.
The link I posted above will help you with this, but in principle the minimum income is based on an annual gross earned income of £18600

You only need to be informed about the meaning of adequate income IF you are exempt from the Financial Requirement.

Read the thread I give and follow all the links. Ask any specific questions you may have.

2. The Fiance(e) visit is a 6 month duration visa issued solely for the purpose of marriage in UK. There are a lot of visa conditions and restrictions.
The major restriction of the holder of a Finace(e) visa is that of being prohibited from any form of work (paid or unpaid) or that of participating in any kind of
business activity (paid or unpaid) or in any kind of voluntary activity/placement.

This can become a major issue. However to minimise the restrictions, once the marriage have taken place the the FLR(M) Further Leave to Remain spouse
visa may be applied for in person at a UKBA enquiry office (by appointment) if you are able to pay the additional premium fee.
Without doing this the waiting time for FLR can be quite a long time (8-10 months possible)

Hope this will help your researching
Good Luck