Following is the list of performance and scalability differences between SQL Server 2014 and SQL Server 2008 (ref. Technet). The most important feature is called in-memory OLTP. You can download and read the white paper (.pdf) from here, which describes the internals of in-memory OLTP.
PERFORMANCE and SCALE
- In-Memory OLTP
- Enhanced In-Memory ColumnStore for DW
- Support for 640 logical proc. & 4 TB memory
- Support to 15,000 partitions
- Resource Governor IO governance
- Buffer Pool Extension to SSDs
- Query optimization enhancements
- SysPrep at cluster level
- Predictable performance with tiering of compute, network, and storage with Windows Server 2012 R2
- Data Compression with USC-2 Unicode support
- Backup Compression