Transaction Log backups LSN values

Hi,

Do anyone has good understanding of TLOG backups[.trb files] which are created as part of SMSQL.

I need some info on how the first LSN,Last LSN values are calculated and how this effects the log chain.

In what cases can the chain break..

I have a situation where 3 log backups has same First LSN and different Last LSN.what is this mean?

Thanks,

Hemanth