Viruses like SARS-CoV-2 continuously evolve as changes in the genetic code (caused by genetic mutations or viral recombination) occur during replication of the genome.

In late 2023, the World Health Organization listed nine variants as circulating at the time. More than 50 variants have been identified; although some are no longer spreading. COVID Variants | WebMD

Some variants have different behaviors.
For example Omicron BA.2 infects different cells in the upper respiratory tract rather than in the lungs, which makes it more transmissible but less severe than the previous variants.
The death rates were slightly lower even though the infection rates 4 or more times higher, so the death per infection were about ¼ the rate of previous variants.
Delta caused more severe disease than other variants in people who weren't vaccinated.

The World Health Origination (WHO) and The European Centre for Disease Prevention and Control (ECDC) classify some variants with concerning properties likely to impact the epidemiological situation in the European Union (EU) and European Economic Area (EEA) as variant under monitoring (VUM), variant of interest (VOI) and variant of concern (VOC).
XBB.1.5-like (Omicron) and BA.2.86 (omicron) are classified as VOI in May, 2024.
Other variants have been de-escalated e.g. ALpha (B.1.1.7), Delta (B.1.617.2), Omicron (BA.1,BA.3, BA.2.L452x)

SARS-CoV-2 variants of concern as of 31 May 2024 | ECDC

New variants emerge in different places.

SARS-CoV-2 Variant Classifications and Definitions

From Alpha to Omicron: How Different Variants of Concern of the SARS-Coronavirus-2 Impacted the World | PubMed - NIH

SARS-CoV-2 Omicron variants: burden of disease, impact on vaccine effectiveness and need for variant-adapted vaccines