A series of transaction hashes and a header. Not the whole transactions!!!!!
The blocks themselves are simply an ordered list of transaction hashes and a header plus some other information. The transactions themselves are not included when the block is propagated. It is assumed that all miners will have the transactions cached and validated, making the block validation and propagation very fast.
The block includes the following:
1: The magic number that defines this as a Bitcoin block on the network it has been mined for
2: The block size in bytes including the full size of all transactions (This is not what is propagated)
3: The block header which contains:
a: The block version number
b: A reference to the previous block which this block builds upon
c: The Merkle root which is the top of a hash tree of all of the transactions in that block
d: A timestamp of when the block was found
e: Difficulty required to have a valid block hash
f: A nonce which when hashed with the information in the header creates a block hash with the right difficulty
4: The qty of transactions in the block
5: The list of all the transaction hashes in the block