A thin client is a type of Bitcoin wallet software that does not operate as a P2P network node or download the entire blockchain. Instead, it tracks Bitcoin transactions by querying a single peer server on the Bitcoin network only for the transactions that involve the addresses in its wallet. This functionality is known as Simple Payment Verification (SPV).
While more susceptible to double spend attacks, thin clients are far more efficient in terms of disk space storage, bandwidth, and time required to verify transactions. This improved performance makes thin clients a more appropriate Bitcoin wallet for use on devices such as mobile phones.
However, most thin clients do not fully protect the user’s privacy: they reveal the user’s IP address and the addresses which belong to the user’s wallet to the SPV server.
In comparison, a “full client” downloads and synchronizes the entire blockchain on your computer. At the time of this writing, the size of the blockchain is over 16GB.