for example, I have a raw sequence .fastq that is trimmed, QC-ed and bowtie2 mapped to a reference genome. to count the reads in the .bam file:

samtools view -c abc.bam (Print only the count of matching records)
12414891
 samtools view -c -f 4 abc.bam (unmapped reads)
191787
samtools view -c -F 4 abc.bam (mapped reads)
12223104

so the mapped reads + unmapped reads = matching records

what does it mean if a read is matching but not mapped?

Also the total read # is a .bam is clearly less than the total read number of QC-ed fastq, meaning there are a lot more unmapped reads in .fastq. what is the difference between unmapped reads in .bam and unmapped reads in fastq? (if I have a read of (T)150, will this be included by -f 4?)