what is exactly three fold repetion

Do both players have to make same 3 moves or only the one trying to force the draw? What is the exact rule?

Nothing to do with the same moves.

Everything to do with the same position.

Same position not only means all the same pieces on all the same squares. But also the same player to move and all the same legal moves (like en passant or castling).

It's unlikely to happen of course, but if the same position occurred on moves 10, 20, and 47 you could claim a draw. 

No. All the same types of pieces.

If two pieces of the same type (two rooks or two knights, or in case of promotion two queens or two bishops on the same colour) have interchanged positions through the moves in the meantime, it counts as the same position.

After a promotion it can't be the same position as the pawn is not there anymore.

For interchanged Knight is "same kind and colour occypy", so yes is the same position!

Here' the part or the rule 9.2 from law of chess.

Positions are considered the same if and only if the same player has the move, pieces of the same kind and colour occupy the same squares and the possible moves of all the pieces of both players are the same. Thus positions are not the same if:

1. at the start of the sequence a pawn could have been captured en passant. 
2. a king or rook had castling rights, but forfeited these after moving. The castling rights are lost only after the king or rook is moved.

Sure. What I meant is that interchange of 2 queens of the same colour can only happen after promotion, because without promotion there is only one queen of the same colour. Same applies to bishops - they are colorbound and start at different colours, so unlike knights or rooks cannot be interchanged, unless by promotion a player does acquire two bishops on the same colour.

So it's "piece of same kind and colour occupy the same square"

In ths case yes it's same position.

What interests me is the "pawn could have been captured en passant thing" and the "forfeit castling rights". This is complicated. A position with castling rights is not the same as one without. A position where a pawn can be captured en passant is not the same as one where it cannot be.

That's right, It's not the same position.

Note that the squares must be actually the same.

If there are no pawns or bishops on board, then the chessboard is symmetric to rotation and reflection, and there can be 8 reflections of each position, which are completely equivalent for attack and defence strategies and prospects - yet count as different positions for purposes of threefold repetition. If bishops exist, then each position still has 4 reflections.

You start your post with "no" but then you don't disagree with me...

Oh I see, what you mean, never mind. That's what I meant.

I do disagree. If two pieces of the same type and colour exchange positions, are they "same pieces"? It´s important to specify that they are.

You're right. I should have said something like: if two positions have the same set of legal moves then they're counted as the same.

3-Fold Repetition is:

• The Same Position
• With the Same Player To Move
• Said Player Having the Same Legal Options
• Occurs 3 Times

See below:

No - both players having the same legal options. Loss of a potential castling right by the other player also makes a position different.

Note that it´s potential castling right, not present castling right. If you compare positions where a king does not have a present castling right because of an attack on a relevant square then it still makes a difference as to whether a potential castling right has been lost ot not.

I'm sorry Thriller-fan, your example is not correct, 25th and 27th differs than 38th because now the black can't castle.

No, you are wrong.  Take the following:

WRONG!  It's White to move - Black's legal moves are irrelevant because Black has no legal moves, it's not his turn!

are you sure?

Here' the part or the rule 9.2 from law of chess.

Positions are considered the same if and only if the same player has the move, pieces of the same kind and colour occupy the same squares and the possible moves of all the pieces of both players are the same. Thus positions are not the same if:

"...the possible move of all the piece of both players..."

I insist: "25th and 27th differs than 38th because now the black can't castle."

I agree with Ciak.  The idea behind threefold repetition is that neither side has made any progress towards winning (or losing).  Black losing the right to castle means that White has potentially made progress towards winning, and is therefore a different position regardless of whether you are looking at White to move or Black to move.

Ciak and omnipaul are correct. A convenient way to remember in this day of computers is that a position counts as a repetition if the FEN is the same (except move number.) i.e.

- Board position. Basically what you see.
- Side to move. (w or b)
- Castling rights of both sides. (KQkq)
- En passant rights.

The castling rights of both sides matters, because it may affect the outcome of the game with sensible play, constituting "progress". To take a crude example, with white to move this is either a clear mate in 2 or a perhaps drawable situation for black, depending on whether black can castle: