Television picture resolution refers to its vertical and horizontal detail.  Vertical-resolution is the number of vertical-transitions, the number of horizontal-lines, traced across the screen top to bottom.  Horizontal-resolution is the number of horizontal-transitions, the number of pixels per horizontal line.

Analog Television

NTSC TV

The original analog NTSC TV system had up to 525 lines of vertical resolution.  43 of its lines included horizontal & vertical synchronizing data.  That left a balance of 482 lines for the image.

However, the NTSC analog system did not produce all 482 lines sequentially or progressively.  Instead, NTSC used an interlaced method of two alternating fields to compose one video frame.

The first field traced the odd-numbered lines in 1/60th of a second.  The second field traced the even-numbered lines in the next 1/60th of a second.  NTSC completed a frame in 1/30th of a second, fast enough for the brain to perceive the video as a complete frame of a moving image.

As stated, NTSC offered up to 482 lines of vertical resolution.  However, the vertical resolution varied as interlaced scenes transitioned from one to the next.  For example, a transition from all black to an all-white frame reduced the vertical resolution to 241 lines.  NTSC could only produce all 482 lines during a still picture frame.

NTSC video interlacing determined actual vertical resolution, while the video source determined horizontal resolution. 
The DVD offers the best NTSC horizontal resolution, 720 pixels per line.  LaserDisc horizontal resolution equaled 400 pixels, VHS videotape 240 pixels.

Digital Television

SDTV

Standard Definition Television, SDTV, has a vertical resolution of up to 480 interlaced lines.  Therefore, SDTV retains the same vertical resolution interlaced issues as NTSC TV.  But the absence of analog noise is an improvement over NTSC.  SDTV horizontal resolution ranges from 640 to 704 pixels per line.  SDTV is a clear improvement over NTSC.

EDTV

Enhanced Definition Television, EDTV, is a significant improvement over NTSC and SDTV.  EDTV produces the video progressively line by line, which eliminates interlace issues.  EDTV offers a fixed vertical resolution of 480 lines by 704 pixels per line.

HDTV

High Definition Television offers three resolution options.
-- 720p by 1280   @ 60 frames per second with a 16:9 aspect ratio.  (broadcast)
-- 1080i by 1920  @ 60 frames per second with a 16:9 aspect ratio.  (broadcast)
-- 1080p by 1920 @ 60 or 24 frames per second.                               (non-broadcast)

720p (progressive) has a vertical resolution of 720 lines with a horizontal resolution of 1280 pixels per line at 60 frames per second (fps).

1080i (interlaced) results in a vertical resolution ranging from 540 to 1080 lines with 1920 pixels per line at 30 frames per second.

720p vs 1080i
The number of pixels per second is almost indistinguishable between 720P & 1080i video. 
However, sports broadcasters have favored 720p because it eliminates interlacing artifacts.

1080p high definition non-broadcast sources -- Blu-ray disc, computer media, and Internet streaming -- eliminate the interlacing issue.

All HDTV sources offer a broader palette of color than NTSC, SDTV, or EDTV.

UtltraHD

UltraHD TV (UHD) increases video resolution to 2160p by 3840 pixels per line @ 30fps, 60fps, and a planned 120fps with a 16:9 aspect ratio. 
UltraHD also doubles the color gamut of HDTV.  UltraHD is often incorrectly referred to as 4K TV.  Some also refer to UltraHD as UHD-1.

DCI 4K Standards

DCI is the real 4K standard.
The Digital Cinema Initiative (DCI) defines digital theater standards which includes: 
- 4K DCI 2.39:1 aspect ratio with 1716 lines by 4096 pixels per line.
- 4K DCI 1.85:1 aspect ratio with 2160 lines by 3996 pixels per line.

8K UltraHD 

8K UltraHD (UHD-8K) doubles UHD resolution to 7680p lines by 7320 pixel per line. UHD-8K televisions are currently available.  

However, 8K sources are not yet readily available. ( January 2022)  Some refer to 8K UltraHD as UHD-2.

UltraHD/UHD-8K prerequisites 

UltraHD/UHD 8K includes HDMI encryption prerequisites.  
Their absence will lead to downgraded video, noisy picture, or a blank screen. 

1. HDMI 2.0 / 2.0a / 2.0b / 2.1 / 2.1a

    HDMI is an interconnect encryption system.  Page 9 offers more HDMI explanation.

Many 1st generation UltraHD televisions and video components are limited to HDMI 1.4 @ 30fps.

2. HDCP 2.2 / (8K) 2.3

High-bandwidth Digital Content Protection encrypts digital video sources with a key that requires authentication from both the transmitting and receiving devices.  HDCP 2.2 is required for all digital consumer -- HD/UltraHD -- video devices.  HDCP 2.3 is an 8K video standard.  Page 9 offers more explanation.

3. MPEG Compression 

4. HDR

High Dynamic Range (HDR) expands color volume.  Page 9 covers HDR formats in detail.    
HDR is considered by many as the most significant television improvement since the introduction of color. 

Video Scaling

A TV displays a blurry mess if the source resolution does not match the TV resolution.  This became a widespread issue with the launch of HDTV in 1988.    NTSC source resolution did not align with HDTV.

An NTSC DVD consists of 345,600 pixels, while an HDTV consists of 2,073,600 pixels.  They don't align.  Then HDTV does not align with UltraHD 4,147,200 pixels.  UltraHD does not align with 8K UHD 8,294,400 pixels.  How can you view a DVD on a HD, UHD or 8K TV?

Scaling resolves the issue.  Scaling up-converts the source resolution to interface and align with the TV resolution.

Initial types of scaling were essentially a math process.  They primarily divided the source pixels by the display resolution.  For example, it divided the DVD's 345,600 pixels into HDTV's 2.073,600 pixels. 

Scaling then improved with adjacent pixel assessment.  Instead of solely dividing a pixel into more pixels, better faster processors compared adjacent pixels, then filter-adjusted their sharpness.  The improved scaling created better clearer images. 

Further improvement added multiple frame assessment to the adjacent pixel adjustment.  It samples video frames, then selects a filter process from a set of  predefined solutions.  The result reduces motion blur and noisy artifacts. 

The latest methods add a version of artificial intelligence.  AI scaling adds an extensive reference image database.  In real-time, the scaling computer/processor compares each frame to the database of images and then applies the best adjustment solution. 

Scaling methods are available in many proprietary versions.  The best solutions separate the best manufacturers from the others.