What's the difference between a sequence and a collection in Clojure

Any object supporting the core first and rest functions is a sequence.

Many objects satisfy this interface and every Clojure collection provides at least one kind of seq object for walking through its contents using the seq function.

So:

user> (seq [1 2 3])
    (1 2 3)

And you can create a sequence object from a map too

user> (seq {:a 1 :b 2})
    ([:a 1] [:b 2])

That's why you can use filter, map, for, etc. on maps sets and so on.

So you can treat many collection-like objects as sequences.

That's also why many sequence handling functions such as filter call seq on the input:

 (defn filter
  "Returns a lazy sequence of the items in coll for which
  (pred item) returns true. pred must be free of side-effects."
  {:added "1.0"
   :static true}
  ([pred coll]
   (lazy-seq
      (when-let [s (seq coll)]

If you call (filter pred 5)

  Don't know how to create ISeq from: java.lang.Long
                  RT.java:505 clojure.lang.RT.seqFrom
                  RT.java:486 clojure.lang.RT.seq
                 core.clj:133 clojure.core/seq
                core.clj:2523 clojure.core/filter[fn]

You see that seq call is the is this object a sequence validation.

Most of this stuff is in Joy of Clojure chapter 5 if you want to go deeper.

Here are few points that will help understand the difference between collection and sequence.

1. "Collection" and "Sequence" are abstractions, not a property that can be determined from a given value.

2. Collections are bags of values.

3. Sequence is a data structure (subset of collection) that is expected to be accessed in a sequential (linear) manner.

The figure below best describes the relation between them:

You can read more about it here.

Every sequence is a collection, but not every collection is a sequence.

The seq function makes it possible to convert a collection into a sequence. E.g. for a map you get a list of its entries. That list of entries is different from the map itself, though.