Cancer classification and diagnosis rely on understanding tissue origins, cellular behavior, and molecular alterations. Human tissues arise from three embryonic germ layers, which determine lineage-restricted cancer types. Because epithelial tissues undergo lifelong turnover and environmental exposure, carcinomas account for the majority of adult cancers, whereas sarcomas, blastomas, and other nonepithelial tumors are far more common in children. Modern classification integrates histology with genomic and epigenomic features, enabling subtype-specific prognosis and treatment decisions. Traditional tumor diagnosis combines clinical evaluation, imaging modalities, laboratory tests, and tissue-based pathology. Computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), single-photon emission computed tomography (SPECT), ultrasound, and endoscopy characterize solid tumors, but definitive diagnosis requires histopathological examination. Hematologic malignancies instead depend on blood and bone marrow smears, fluorescence-activated cell sorting (FACS), and molecular assays. Increasingly, comprehensive genomic profiling at diagnosis identifies targetable alterations and supports precision therapy. Liquid biopsies provide minimally invasive monitoring of clonal evolution and relapse risk. Screening strategies such as mammography, colonoscopy, Pap smears, and low-dose CT detect tumors before symptoms emerge, which improves survival. Solid tumors grow as localized masses shaped by their microenvironment, while hematological cancers disseminate early through blood and lymphatic systems. Adult cancers typically accumulate mutations over decades, whereas pediatric tumors arise from developmental or epigenetic dysregulation. Sex differences in hormones, genetics, and immunity further influence cancer susceptibility and therapeutic outcomes.