web3 contains an eth object (web3.eth) specifically for Ethereum blockchain interactions and an shh object (web3.shh) for whisper interaction. Most APIs of web3.js are inside these two objects.

All the APIs are synchronous by default. If you want to make an asynchronous request, you can pass an optional callback as the last parameter to most functions. All callbacks use an error-first callback style.

Some APIs have an alias for asynchronous requests. For example, web3.eth.coinbase() is synchronous, whereas web3.eth.getCoinbase() is asynchronous.

Here is an example:

//sync request 
try 
{ 
  console.log(web3.eth.getBlock(48)); 
} 
catch(e) 
{ 
  console.log(e); 
} 

//async request 
web3.eth.getBlock(48, function(error, result){ 
    if(!error) 
        console.log(result) 
    else 
        console.error(error); 
}) 

getBlock is used to get information on a block using its number or hash. Or, it can take a string such as "earliest" (the genesis block), "latest" (the top block of the blockchain), or "pending" (the block that's being mined). If you don't pass an argument, then the default is web3.eth.defaultBlock, which is assigned to "latest" by default.

All the APIs that need a block identification as input can take a number, hash, or one of the readable strings. These APIs use web3.eth.defaultBlock by default if the value is not passed.