Meaning and use
We can use the present continuous to talk about the future when we have already arranged to do something. It's definite, not just an idea. Perhaps, we have put the arrangement in our diaries.
'Are you free at 2 o'clock on Tuesday?' 'No, I'm meeting a client.'
We can use be going to to talk about something that we have decided to do in the future, but not arranged yet. The plan might change.
When I finish college, I'm going to spend a year travelling.
The uses of the present continuous and 'be going to' with future meaning are very similar and it is often possible to use either of them, with little or no change in meaning.
We also use be going to for something that we expect to happen because we can see from the present situation that it is very likely. We can't use the present continuous in this way.
It's so cold. I'm sure it's going to snow soon.
Oh no! That car's going to hit the tree.
Form
Present continuous
Subject + am/is/are + -ing form
Be going to
Subject + am/is/are + going to + infinitive
For both tenses, we usually use contractions or short forms (I'm, he's, we're, etc.) when we are speaking.
Positive
I'm spending the day on the beach tomorrow with Zach.
We're going to look for a nice restaurant with a view of the sea.
Negative
We're not visiting the museum on Saturday.
They're not going to visit the Taj Mahal.
Question
What are you doing at the weekend? (= What have you arranged?)
What are you going to do at the weekend? (= What is your plan?)
Are they going to get married in the summer?
Take note: Time expressions
For both the present continuous for arrangements and 'be going to' for plans, we often use time expressions like at half-past ten, next Thursday, at the weekend, soon.
Are you coming home soon?
I'm going to send out all the invitations next week.
Take note: arrangements and timetables
Use the present continuous and NOT the present simple for things that you have arranged to do. Use the present simple for future events on timetables and programmes.
We're going by train tomorrow morning.
The train leaves at 7.45.
http://www.bbc.co.uk/learningenglish/english/course/lower-intermediate/unit-9/tab/grammar
