Viruses are DNA or RNA in a capsid (protein) shell

They are not considered "alive" because they do not do anything that a living thing requires. No organs (organelles in microbes), do not carry out any metabolic processes (no eating, growing, use energy, etc), and the biggest thing in microbiology class they do not reproduce without needing a host.

The reason the non reproducing thing is the biggest point is because a lot of other microbes are kind of similar to viruses in the sense of: very little to no organelles (usually have ribosomes), little to no metabolic processes at points in its life cycles. But all microbes can reproduce without a host (such as through binary fission). Viruses need a cell to act as its baby factory, hijack it's organelles (ribosomes usually) to make copies of its DNA or RNA

Side notes: ribosomes are organelles (organs) that take rna (instructions) and make protein (building block of life) (making protein is called protein synthesis, this is called the central dogma of biology). Viruses can use these to make new viruses (since they can't make protein themselves).

A lot of simple bacteria have only dna and ribosomes (so little to almost no organelles)

Binary fission is what you typically think of when cells split to make 2 new cells

DNA is double helix, RNA is a single strand, they get their different names based on their sugar base on the strands. Ribonucleic acid (ribose sugar) deoxyribonucleic acid (derived from ribose but lacks a oxygen (hydroxyl) in it). DNA is basically the "master instructions" to make a cell/living thing, RNA is a more compressed version of it (basically a smaller copy).