This very comprehensive page from the Computer Science Field Guide has extensive but clear notes on lossless and lossy compression. The page is well written and designed for a high school audience, with easy to understand examples, video, and even interactive sections. There are also extension "Extra for Experts" sections. The page covers image compression, audio compression, and text compression.

A short Computerphile video which explains basic compression techniques and how they can be applied to text files and image files. The video also addresses the differences between lossy and lossless compression.

This video explains compression techniques in the context of images. It is recommended that students have an understanding of how image data is stored (e.g. bit depths) before watching this video. The video is very comprehensive and so it may take a couple of viewings to fully understand the details of the examples being presented, but it is worth it.

These videos from Computer Science Tutor make great revision sources. This particular videos covers the theory behind the two types of compression - lossless and lossy.

The National Codes Centre at Bletchley Park (who know a thing or two about ciphers) have comprehensive lesson plans, teacher's notes, and student activities for various types of encryption, from simple substitution ciphers (Caesar ciphers) through the infamous Enigma and Lorenz ciphers to modern day methods.

Very detailed but clear explanations of the various types of encryption systems, from simple substitution ciphers (Caesar ciphers) to modern public key encryption systems. The page also explains the types of attacks that can be performed against ciphers, such as known plaintext attacks and frequency analysis attacks. A variety of interactive applets held improve understanding.

This is a kinesthetic lesson designed to help students understand how public key encryption systems work, as well as the types of security challenges such systems address. The plan includes a list of required materials and a video of the lesson being taught (this is a great help in understanding how things are supposed to work). The lesson plan is quite complicated and so is probably better suited to older students, but the analogy (padlocks) is a great way of explaining what can be a fairly confusing concept.

A short video from Computerphile which explains the weaknesses of symmetric key encryption systems and how asymmetric key (public key) encryption solves them. The video explains how public and private keys can be used for secure communication, and how the private key can be used to authenticate the sender (digital signing).

Braingle contains extensive notes on many different types of cipher systems from Caesar ciphers to four-square ciphers. There are dozens of examples and some interactives to help students understand the different ways plaintext can be encrypted. This could be a good site to support a lesson starter activity or extension activities for more advanced students who are interested in encryption systems.

A good overview of hashing algorithms and where they are used. The video also examines the basic requirements of hash algorithms, including the avalanche effect and hash collisions, and why they are important for security.

A good overview of database theory, which is sufficient to cover the syllabus for most GCSE courses, and is a good introduction to the topic for A Level and IB courses. The differences between structured and unstructured data are covered, along with database basics (fields, records, tables). The second half of the video explains relationships in more detail, including 1-to-1 and 1-to-many relationships.

An animation showing how DNS works when you type the URL of a website into your web browser. This video goes into more detail than some of the others here, showing how a computer locates nameservers before finding the IP address of a domain name. Useful for older students.

This video goes into a LOT of detail about DNS and the different types of nameservers: the root nameservers, the TLD nameservers, and authoriative name servers. This is probably more detail than most students will need, but it can help understand the bigger picture, so is included here.