Each entry has a serial number, a revocation date and optionally a reason code (keyCompromise, superseded, cessationOfOperation and others). The list carries thisUpdate, nextUpdate, a monotonically increasing cRLNumber and the issuer's signature. Large CAs shard their CRLs; certificates point to theirs through the CRL Distribution Points extension, and public CAs also disclose their full set of CRL URLs in CCADB.
Under the Baseline Requirements, a public CA must publish a new subscriber-certificate CRL at least every seven days (every four days if not all of its certificates carry an OCSP URL) and within 24 hours of a revocation. Browsers do not fetch CRLs per connection; they aggregate them into compact sets (Firefox's CRLite, Chrome's CRLSets), which is why CRLs are now the primary revocation channel of the web PKI.
CRLs can be large and their freshness is bounded by nextUpdate, so clients that cache them see revocations late. The CRL decoder lists the entries of a CRL file.