IMPORTANT:

Bitcoin  Core is a full Bitcoin client and builds the backbone of the network.
It offers the highest levels of security, privacy, and stability.
However, it has fewer features and it takes a lot of space and memory.

Bitcoin Core initial sync can take a very long time to complete.
You should make sure that you have enough bandwidth and storage for the
full block chain size. In March 2014, this zize is 15 GB, see
http://blockchain.info/charts/blocks-size

If you know how to download a torrent file, you can speed up this process
by putting bootstrap.dat (a previous copy of the block chain) in the
Bitcoin Core data directory before starting the software.

Torrent URL for bootstrap.dat is:
http://sourceforge.net/projects/bitcoin/files/Bitcoin/blockchain/bootstrap.dat.torrent/download