Postgres is the predecessor of PostgreSQL. Postgres used a different query language when it was still a university project led by Stonebraker. Postgres was the next project after Ingres as the name suggests, and its query language was originally similar...called QUEL instead os SQL or something like that.
Postgres forked into two code bases after the university project ended. Stonebraker started a company called Illustra to sell a commercial version of Postgres. Informix eventually bought Illustra and called it Online Dynamic Server if i recall, and by the time IBM bought informix this sibling of Postgres was the flagship product.
The second fork of postgres was picked up by former students of Stonebraker (initially Joly Chen if i recall and one or two others..too lazy to google for the details). They introduced a SQL parser front end of their own and called the initial release Postgres95 v1.x since it was the fad MSFT started to use years in product names, and also resetting the version number given the changes in features and management (postgres was at version 4.x).
When it came time to release the next major version the name was looking dated and redundant since there was still a release number. So the name became PostgreSQL as it was more meaningful (the primary feature difference being the query language). The version number was then "un-reset" too...postgres95 1.x being considered as 5.x and the first PostgreSQL named release being 6.0.
So yes, postgres isn't the same as postgresql. It is mist accurately described as the father of PostgreSQL and Informix. Architecturally the latter two are essentially the same, but their SQL parsers are unrelated as they were each developed post fork, plus the codebases diverged quite significantly over the past 20 years.
They are both fantastic databases by the way...they wipe the floor with mysql. To say postgreSQL is not web scale are ignorant and probably last used it in the 1990s if at all. It truly kicks butt for full text search, geospatial data for mapping or survey data, astronomy and so on. It is 10+ years ahead of Microsoft SQL server or mysql at that stuff as well as things like multi version concurrency...i was spoiled by PostgreSQL MVCC when i had to contend with rows and tables being locked until transactions wete committed in other RDBMSes.
MySQL has no extensibility, nowhere near the rich set of data types or extensibility, and is not optimised for write heavy ACID transaction stuff. MySQL is great for your CD collection or your blog or whatever, but PostgreSQL is still far superior for accounting/erp/mapping/etc, though i do acknowledge MySQL/MariaDB has gotten "good enough" it is far frpm the best.
And dont start with me on noSQL. Its a great hammer but only some applications are nails, even at "web scale".
Congratulations to Dr Stonebraker. His legacy in the industry is impressive and his work has led to a Free database project that can truly take on the big O on many serious fronts.