Query: How Much X for Y?

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Overview
In many scenarios, you might want to know how much X of TokenA you'll receive for Y of TokenB. Fortunately, Balancer has query functions to simulate transactions.
If you look on Etherscan (or similar), all three of the query functions will show up as "Write" functions. That is ok! You can still call these with eth_call to get numerical results without spending any gas.
!⚠️! WARNING !⚠️!
Don't call query* from a contract!!!
If you are using queryBatchSwap, queryJoin, or queryExit to calculate limits,  maxAmountsIn, or minAmountsOut, this is ONLY useful if you simulate these calls OUTSIDE of the transaction you end up making. You SHOULD NOT call these functions from a smart contract at transaction time.
Why?
Calculating these values ahead of time is useful for enforcing slippage tolerances to mitigate losses due to sandwich attacks. If you are querying these values at execution time, you end up getting your values mid-sandwich! This leaves you entirely vulnerable to the attacker's manipulation, and is as foolish as using a 100% slippage tolerance.
Queries
Are you trying to calculate amounts for a pool that uses Phantom BPT?
*LinearPools and StablePhantomPools do not have join or exit functionality since those are handled as swaps! For example, if you want to figure out how much bb-a-USD you'll get for an amount of DAI, you'll need to use queryBatchSwap on a trade route that swaps DAI for bb-a-DAI and then swaps bb-a-DAI for bb-a-USD. 
queryBatchSwap
To calculate the inputs/outputs for a trade (you can specify given-in or given-out), you will use the queryBatchSwap function in the Vault. This functionality is important if not crucial for calculating your limits when constructing your batchSwap arguments.
queryJoin
To calculate amounts of BPT out and tokens in, you will use queryJoin in BalancerHelpers. This functionality is important for calculating maxAmountsIn and/or minBptOut on joins 
queryExit
To calculate amounts of BPT in and tokens out, you will use queryExit in BalancerHelpers. This functionality is important for calculating minAmountsOut and/or maxBptIn on exits.