HDMI线是高清晰多媒体接口线的缩写，能高品质地传输未经压缩的高清视频和多声道音频数据，最高数据传输速度为5Gbps。
HDMI线根据接口的可以分为三类：
HDMI A TYPE：标准HDMI接口，宽约15MM, 高约5MM；
HDMI C TYPE：Mini HDMI接口，宽约10.5MM, 高约2.5MM；
HDMI D TYPE：Micro HDMI接口，宽约6MM, 高约2.3MM，手机等数码设备高清接口。
HDMI接口类型从版本区分：
版本HDMI 1.0 频率165MHZ 带宽4.95Gbps 原始标准，最近已基本淘汰；
版本HDMI 1.1 频率165MHZ 带宽4.95Gbps 增加DVD-Audio支持等；
版本HDMI 1.2 频率165MHZ 带宽4.95Gbps 增加SACD音频支持等；
版本HDMI 1.2a 频率165MHZ 带宽4.95Gbps 增加兼容性认证等要求；
版本HDMI 1.3 频率340MHZ 带宽10.2Gbps 提高通讯带宽，增加TrueHD和DTS-HD音频；
版本HDMI 1.4 频率340MHZ 带宽10.2Gbps 增加一条数据通道，支持百兆以太网双向通信。
版本HDMI 2.0 带宽为18Gbps，而上一版的HDMI1.4带宽则为10.2Gbps，提升较大。
版本HDMI 2.1 标准版支持带宽最大为48Gbps，可以支持4K/120Hz、8K/60Hz、10K/120HZ等分辨率和刷新率的无损视频。
除了这些新特性，HDMI 2.1还针对视频、电影、游戏增加了多个增强刷新率技术，保证用户在使用高分辨率时流畅无卡顿现象，比如：
用于游戏的“可变刷新率(VRR)”，可减少画面的延迟、卡顿、撕裂的现象，让玩家在进行游戏时更加的流畅。
还有用于视频和电影的“快速媒体切换(QMS)”，可消除内容显示前导致白屏的延迟。
而“快速帧传输(QFT)”则是针对VR虚拟现实，大幅降低延迟，让实时交互的体验更加好。
此外还有“自动低延迟模式(ALLM)”，在这个模式下可以自动设定最佳延迟。
第一：提升画质
HDMI绝对的强，而在入门级DVD机如果加入新的组合数码接口，可以让入门DVD机在画面质量上有可与中级机比拼的筹码。使用高清除HDMI外，机身的色差输出一样可对应720p或1080I论画质，不过色差输出很注重线的质量，而HDMI对线的要求没有色差那么高，贵线与普通线的效果差距不是太明显（现在市场上最便宜的HDMI线也就在70元左右而且带屏蔽磁环），很重要的一点就是相对于DVI接口，它对距离的要求信号源与接收端之间的距离不能超过15米。
第二：提升音质
HDMI的最大优点，是既能以无压缩传送全数码的视频信号，亦同时支持数码音频传送，只靠一条HDMI线接驳等离子，就能同时欣赏到清晰细腻的画面，及通透立体的声音。不过现在只有少数厂家推出的机型有支持这种方式，这种播放器的成本也比较高。
第三：HDCP（高速宽带内容保护）
如今的盗版现象在全世界范围内都有，比较严重的地区就在中国和俄罗斯，今后高清晰的片源越来越多，画质好且又几乎不失真，极容易被复制。因此没有HDCP版权保护这个技术。许多制片公司和电影厂是会吃大亏的，尤其是如今的电影大厂都在美国，所以美国人为了自己的利益当然要主动出击，Intel的HDCP就是用来保护这种基于Internet的数字多媒体视频版权的一种技术。在电视、电影以及交互型（互动）网络形式大规模出现之前，Intel能将这个内容保护功能配置在所有数字显示器以及数字视频显示终端上。本回答被网友采纳
楼上的回答很完善了，如果楼主或其他人需要英文版的解释，请看以下信息：
An HDMI cable is composed of four shielded twisted pairs, with impedance of the order of 100 Ω (±15%), plus seven separate conductors. HDMI cables with Ethernet differ in that three of the separate conductors instead form an additional shielded twisted pair (with the CEC/DDC ground as a shield).[51](§HEAC-2.9)
Although no maximum length for an HDMI cable is specified, signal attenuation (dependent on the cable's construction quality and conducting materials) limits usable lengths in practice[68][69] and certification is difficult to achieve for lengths beyond 13 m.[70] HDMI 1.3 defines two cable categories: Category 1-certified cables, which have been tested at 74.5 MHz (which would include resolutions such as 720p60 and 1080i60), and Category 2-certified cables, which have been tested at 340 MHz (which would include resolutions such as 1080p60 and 4K30).[6](§4.2.6)[62][71] Category 1 HDMI cables are marketed as "Standard" and Category 2 HDMI cables as "High Speed".[4] This labeling guideline for HDMI cables went into effect on October 17, 2008.[72][73] Category 1 and 2 cables can either meet the required parameter specifications for interpair skew, far-end crosstalk, attenuation and differential impedance, or they can meet the required nonequalized/equalized eye diagram requirements.[6](§4.2.6) A cable of about 5 meters (16 feet) can be manufactured to Category 1 specifications easily and inexpensively by using 28 AWG (0.081 mm²) conductors.[68] With better quality construction and materials, including 24 AWG (0.205 mm²) conductors, an HDMI cable can reach lengths of up to 15 meters (49 feet).[68] Many HDMI cables under 5 meters of length that were made before the HDMI 1.3 specification can work as Category 2 cables, but only Category 2-tested cables are guaranteed to work for Category 2 purposes.[74]
As of the HDMI 1.4 specification, the following cable types are defined for HDMI in general:[75][76]
Standard HDMI Cable – up to 1080i and 720p
Standard HDMI Cable with Ethernet
Standard Automotive HDMI Cable
High Speed HDMI Cable – 1080p, 4K 30 Hz, 3D and deep color
High Speed HDMI Cable with Ethernet
A new certification program was introduced in October 2015 to certify that cables work at the 18 Gbit/s maximum bandwidth of the HDMI 2.0 specification.[77] In addition to expanding the set of cable testing requirements, the certification program introduces an EMI test to ensure cables minimize interference with wireless signals. These cables are marked with an anti-counterfeiting authentication label and are defined as:[78]
Premium High Speed HDMI Cable
Premium High Speed HDMI Cable with Ethernet
In conjunction with the HDMI 2.1 specification, a third category of cable was announced on January 4, 2017, called "48G".[79] Also known as Category 3 HDMI or "Ultra High Speed" HDMI, the cable is designed to support the 48 Gbit/s bandwidth of HDMI 2.1, supporting 4K, 5K, 8K and 10K at 120 Hz.[80] The cable is backwards compatible with the earlier HDMI devices, using existing HDMI type A, C and D connectors, and includes HDMI Ethernet.
Ultra High Speed HDMI Cable (48G Cable) – 4K, 5K, 8K and 10K at 120 Hz