Category 5:
The original specification for category 5 cable was defined in ANSI/TIA/EIA-568-A, with clarification in TSB-95. These documents specified performance characteristics and test requirements for frequencies of up to 100 MHz.
Category 5 cable includes four twisted pairs in a single cable jacket. This use of balanced lines helps preserve a high signal-to-noise ratio despite interference from both external sources and other pairs (this latter form of interference is called crosstalk). It is most commonly used for 100 Mbit/s networks, such as 100BASE-TX Ethernet, although IEEE 802.3ab defines standards for 1000BASE-T - Gigabit Ethernet over category 5 cable. Cat 5 cable typically has three twists per inch of each twisted pair of 24 gauge copper wires within the cable. May be unsuitable for 1000BASE-T gigabit ethernet.

Category 5e:
Cat 5e cable is an enhanced version of Cat 5 that adds specifications for far end crosstalk. It was formally defined in 2001 in the TIA/EIA-568-B standard, which no longer recognizes the original Cat 5 specification. Although 1000BASE-T was designed for use with Cat 5 cable, the tighter specifications associated with Cat 5e cable and connectors make it an excellent choice for use with 1000BASE-T. Despite the stricter performance specifications, Cat 5e cable does not enable longer cable distances for Ethernet networks: cables are still limited to a maximum of 100 m (328 ft) in length (normal practice is to limit fixed ("horizontal") cables to 90 m to allow for up to 5 m of patch cable at each end). Cat 5e cable performance characteristics and test methods are defined in TIA/EIA-568-B.2-2001.

Connectors and other information:
The cable exists in both stranded and solid core forms. The stranded core has the advantage of being more flexible and withstanding more bending without breaking. The solid core has the advantages of being easier to connect to terminals and less expensive. Taking these things into account, building wiring (for example, the wiring inside the wall that connects a wall socket to a central patch panel) is usually solid core, while patch cables (for example, the movable cable that plugs into the wall socket on one end an a computer on the other) are usually stranded.

Summary:
As a basic rule of thumb, it is always better to buy cat5e cables than the unshielded cat5 cables, mainly due to the price difference being almost negligible. Cat 6 currently does not come in many colours and is still quite expensive when you consider that there isn't too much of an advantage for the general end user over a Cat 5e cable.