Write ahead logging as one of the main principles of ARIES means that any change done to a database object such as inserting, updating, or deleting a row is recorded in a log before it gets offloaded to disk to a physical tablespace. Hence, this technique is called write ahead logging. If you commit your changes, Db2 guarantees the data is saved into a log dataset. The changes do not necessarily need to be mirrored in the tablespace yet. The log in this sense in Db2's perspective is the log buffers (see the OUTBUF ZPARM) and active/archive logs.
Db2 writes log records into the log. IBM provides a mapping macro for those records and you can find the macro in SDSNMACS(DSNDQJ00). The macro is a bit difficult to read, but you don't need to understand it. That is why there are log analysis tools that help to analyze the data and display the relevant information. If you have nothing else, you still have DSN1LOGP utility, which can at least format the log records.
In fact, all is a bit more difficult, because log records are saved in VSAM Control Intervals and log records may even span multiple CIs. You really don't want to do this business all on your own. But if you still want, IDCAMS can be your friend, I've warned you.
Anyway, back to log records. Every log record in Db2 has a log record header. Header possesses important data, such as record type (we will touch some), Db2 version that created this record, some other control information, but most importantly it includes log record sequential number (LSN). LSN which is another building block of ARIES. In Db2 terms, this can be the relative byte address (RBA) or log record sequence number (LRSN). RBA is based on the physical offset in the log if you think about the log as a huge ever growing file, while LRSN is based on a store clock timestamp and is used in a data sharing environment as each member has its own log. RBA's and LRSN's common characteristics are that they are ascending and unique 10 byte numbers.
The data changes that your applications does are sooner or later propagated into the data and index pages of the corresponding table/index spaces. The technique that Db2 uses to update the pages, is called in-place updates. Db2 updates the existing pages, while an alternate technique used in some other RDBMs, called shadowing, would write the updates to the new pages.